Software Update Failed Error for iOS 15.7 on iPhone 8

I had the same problem -- tried to install iOS 15.7 on iPhone 8 and got the "software update failed" error.

Here's what worked: I freed up enough storage space until I had about 9 GB free. That's it.

I originally tried when I had about 4-5 GB free. It didn't install, and I got the exact same error as above.

I tried again when I had about 7 GB free. It still didn't install; same error.

Once I freed up enough storage space (by deleting apps and things) until I had about 9 GB free, then the update installed, no problem.

The update server temporarily lost its communication with your phone during the download process.

Power off the iPhone, wait a minute, then power it up again and try the update again.

If all else fails, you may need to connect the phone to a Mac or PC and run the update that way.

Lawrence Finch in a separate post on the same issue said as follows

“To update over the air from the phone all of these requirements must be met:

• You need at least 6 GB of free storage
• You must be connected to a Wi-Fi network (and not a personal hotspot) unless it is an iPhone 12 or 13 series or an SE 3rd edition with an active 5G network connection
• The phone must be charged to at least 50%, and preferably plugged in so it stays above 50% during the update
• The phone must not be in Low Power Mode
• The phone must not be connected through VPN, and it may be necessary to delete any VPN apps and VPN profiles
• The phone cannot be in Airplane mode
• You cannot still have a beta version installed on the phone
• You cannot have a previous version that was downloaded, but not installed - go to Settings/General/iPhone Storage to look; if you find an iOS installer, delete it”

As soon as I got 6 GB of space, 15.7 downloaded and I could install it.

Thank you DeadRedPanda! After getting free space above 6GB, update 15.7 successfully downloaded and installed. I do find this an odd requirement, though, considering the update was only 326MB. And it would be helpful if Apple would add this requirement to the error message.

It finally worked for me. iOS 15.7 downloaded fine and installed successfully on my iPhone X.

The trick is to clear more space and ensure that you’ve at least 10 to 12 GB of free space. This is weird because the download is under 335 MB. But, once I cleared more space and brought it to about 11 GB, everything worked magically!!

It is the storage capacity on your phone, I'm convinced. I earlier posted that but backed off of it because several people said that they had "plenty of storage". My iPhoneX had 3.2 gigs of space and repeatedly failed, despite the update being only a small fraction of that. However, I cleared a lot of photos/videos and increased the space to 19.4 gigs and 15.7 updated easily. The update must have to store a lot of the iOS while it is updating is my guess. Another person of this forum had similar experience by increasing his space drastically. BTW, I use Simple Transfer Pro App to transfer photos. Yes, the Pro version has a one-time cost, but works so superbly and allows unlimited numbers of photos and videos to be transferred. Free version only allows 50 at a time.

To update over the air from the phone all of these requirements must be met:

• You need at least 6 GB of free storage; more is better
• You must be connected to a Wi-Fi network (and not a personal hotspot) unless it is an iPhone 12 or later with an active 5G network connection
• The phone must be charged to at least 50%, and preferably plugged in so it stays above 50% during the update
• The phone must not be in Low Power Mode
• The phone must not be connected through VPN, and it may be necessary to delete any VPN apps and VPN profiles
• The phone cannot be in Airplane mode
• You cannot still have a beta version installed on the phone
• You cannot have a previous version that was downloaded, but not installed - go to Settings/General/iPhone Storage to look; if you find an iOS installer, delete it

If you can't meet all of them you can still update using your computer.

To update over the air from the phone all of these requirements must be met:

• You need at least 10 GB of free storage for iOS 16 or 6 GB for iOS 15 or lower; more is better
• You must be connected to a Wi-Fi network (and not a personal hotspot) unless it is an iPhone that supports cellular 5G (12, 13, 14 or SE 3rd edition) and has a 5G network connection
• The phone must be charged to at least 50%, and preferably plugged in so it stays above 50% during the update
• The phone must not be in Low Power Mode
• The phone must not be connected through VPN, and it may be necessary to delete any VPN apps and VPN profiles
• The phone cannot be in Airplane mode
• You cannot still have a beta version installed on the phone
• You cannot have a previous version that was downloaded, but not installed - go to Settings/General/iPhone Storage to look; if you find an iOS installer, delete it

If you can't meet all of them you can still update using your computer.

Was able to solve it as well: Cleaning up space (about 11 GB free) helped. Seems like the process of checking needed space before installation is broken, or something.

Try to free up storage, that helped to some of us.

In rare cases, having at least 6 Gb of free space is enough to install the update, but I had to clean up 12 Gb to proceed with the update on my SE 2nd gen.

Many say they have “plenty of storage”. Try clearing to a super amount of (15-20 GB) available and try again.

Same issue on iPhone XS. Update wouldn’t download until I had freed up >10GB of storage space. Once that much space was available, update proceeded seamlessly

I was able to resolve this problem by freeing up memory. With under 5Gb free the download failed. With over 10Gb free I had success.

OK, so I had a lot of the same problems with my IPhone 8. Was only at 2Gb capacity, increased it to 5-6GB, dumping photos to PC storage. Then it still sat at “Preparing Update” and i just sat there keeping it from going to sleep and the little blue bar ever-so-slowly went up and completed after 15 agonizing minutes as i read this blog. But it finally did it and it is now updating with white little apple and white bar. Slowly. So. Dang. Slowly.

Using a PC it does a complete reinstallation. The download to iTunes is about 3.5GB. Updating over the air just downloads a patch for “dot” version upgrades. And the download time is controlled by your internet connection speed and nothing else.