Can’t update iPhone

If you have an iPhone 6 or a 5s the highest iOS you can update to is iOS 12.5.5. They do not have sufficient RAM to run iOS 13 or later and you cannot add RAM. You need a newer device that can run iOS 15.5 (currently) and I would suggest you get at least an iPhone 8 as the 6s and 7 series won't run the upcoming iOS 16.

If the device is not compatible with iOS 15, it is not compatible with iOS 14, nor is it compatible with iOS 13.

Exactly which device do you have, and what is the existing version of iOS?