Why is my iPhone 6 not updating

The iPhone 6 cannot update further than iOS 12.5.6 due to hardware limitations.

The iPhone 6S can update to iOS 15.7.2. If you indeed have a 6S and cannot update wirelessly, 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.

As you have provided zero information, it is impossible for us to provide an educated guess.