My iPhone 6 won't update.

If you have an iPhone 6 and it is running iOS 12.5.5, you have reached the end of the road for updates. The iPhone cannot be updated any further than this.

Without a clue, on supported build model version iPhone you've

to begin; can't guess where it may update to; or how far it can go.

A chart of various models in succession, and their supported iOS

may be somewhere to start. You can use this Chart, supplied by

Giulio ~ a helpful contributor, here at Apple iPhone community.

There's also articles to suggest updates; to each and what level.

• Update iOS on iPhone - Apple Support

//support.apple.com/guide/iphone/update-ios-iph3e504502/ios

• iPhone User Guide - Table of Contents (with Supported models) - Apple Support

//support.apple.com/guide/iphone/toc

• How to update to iOS 15 [or iPadOS 15] - Official Apple Support

Because those, over time, were built and later hardware didn't

exist back then. That's why some older models, can't possibly be

updated to a most newest recent one, at this late a time.

Maybe use above chart, to see where your iPhone could be updated?

..Thought to mention: This thread was 're-titled' to remove

accidental "Solved" stamp; & product model, was Added..

Didn't see many visits, posted in 🐯Tiger OS X 10.4 ASC!🙃