My iPhone update fails all the time

What model phone, what do you want to update to?

Go to Settings/General/About and note the iPhone Model Name, then check its version compatibility from this list:

• iPhone 8 or newer, including SE 2nd and 3rd editions, you can update to 16.x
• Any iPhone series 6S and 7, and any model SE: You can update to 15.7
• iPhone 5S or 6: You cannot update to iOS 13 or higher; you can update only to iOS 12.5.6
• iPhone 5: You can update only to 10.3.4
• iPhone 5C: You can update only to 10.3.3
• iPhone 4S: You can update only to 9.3.6
• iPhone 4: You can update only to 7.1.2

After you have established compatibility with the version you want, 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.