updating iphone6

The iPhone 6 can be updated to iOS 12.5.7.

The iPhone 6S can be updated to iOS 15.7.4.

Which model do you have?

If you're running 12.5.7, you do no have a 6s.

12.5.7 is as far as the 6 and 6 plus can go. There is no way to install a later version of iOS on those devices. You will need a new phone.