Why won't my iPhone 6 update to iOS 12.5 5?

If you're on iOS 10 and you want to go to 12.5.5 you will probably need to update via a pc or mac

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.

No, a Chrome Book is not a PC.

If you have an Apple Store or Apple Service shop near you, take the phone in and use a Mac there. Or, you may have a friend with a PC or Mac that would be willing to spend 15-20 minutes of time with you.

What iOS version is currently running on it? (Settings>General>About>Software version)?

What happens when you try to update it?

What model of iPhone do you have?

If you are not sure......Settings > General > About > Model Name.

Edit.......you just marked your post as solved, so you don't need any more help.