iOS 12.7softward update

If you want to install this update, you will need to free up storage space on your iPhone. If this is not working, please update via a pc or Mac instead

Update your iPhone using a Mac with macOS Catalina or higher

On a Mac with macOS Catalina or higher, you will use the finder in order to upgrade your iPhone. Make sure to Back up your device before starting the upgrade process.

On a Mac with macOS Catalina or later, you will follow the following steps:

1. Open a finder window
2. Connect your device to your Mac via a lighting to usb A or a lightning to usb c cable
3. Select your device in the left sidebar of the finder window
4. click general, then click check for update
5. Click download and update.
6. If asked, enter your iPhone passcode

Update your iPhone via a Mac running macOS Mojave or earlier, or on a Windows PC

On macOS Mojave or earlier, or on a windows machine, we won't use the Finder to upgrade your iOS

Instead, we will use iTunes. Make sure to Back up your device before doing any update or upgrade to your phone.

1. Open iTunes
2. connect your device to the computer with an lightning to usb a or a lightning to usb c cable
3. Click device button in the iTunes toolbar
4. In the summary tab, click check for update
5. Click download and update, follow The on screen instructions
6. If asked, enter your iPhone passcode.

Updating/upgrading your iPhone - Apple Community

The message explains things clearly.

You do not have enough free, local storage on your phone to install iOS 12.5.7. You need at least 1.07 GB of free space, and it wouldn't hurt to have slightly more. It's not that the update is likely to need that much extra space forever, but the upgrade process does temporarily require some room to work.

If you want to install the update directly from the phone, you will need to remove some things from the phone to make the necessary space available.

I'm not sure if I would recommend deleting apps to make room - in case you went to reinstall them and found that the iTunes Store did not offer versions that ran on iOS 12.5.7 any longer. You might want to delete music / videos that you loaded onto the iPhone from a Mac or PC, and that you could load onto it again from that Mac / PC.

The other option is to use iTunes (on a computer) to install the update. I'm not sure how much free space you'd need on the iPhone, then, but perhaps not as much.