How to update my iPhone 6

Your iPhone will not update automatically unless you have it set to do that and if your iPhone is eligible for an update. Your iPhone 6 can only update to iOS 12.5.7. You can check by selecting Settings > General > Software Update. If it is eligible for an update, you will be prompted to do so.

Update your iPhone or iPad - Apple Support

There is no supported update for an iPhone 6 past iOS 12.

Your options for apps requiring newer iOS are a newer (used, refurb) or new iPhone.

The iPhone 6, released by Apple 8½ years ago, cannot be updated beyond iOS 12.5.7.

Check Settings > General > Software Update

If the message indicates that your software is up to date, there are no other updates available for your phone.