How to update iPhone 6 to 15.5 ios

Again, the iPhone 6 is not compatible with iOS14.

iOS 13 doesn't appear to be compatible either.

I think the latest version that the iPhone 6 can install is 12.5.5

See these pages for iOS12 versions - https://support.apple.com/en-us/HT209084

iOS 15.5 is not in general or Beta release. iOS15.2 is in Beta, and 15.1 is on general release.

Also the iPhone 6 is too old for these latest releases of iOS. the earliest device that will take iOS15 is the 6S (just not the 6).

You may find that iOS13 is the latest version available for that device.

An iPhone 6 can only update as far as 12.5.5.