how to update iPhone 4

It can not be upgraded further.

the iPhone 4 is limited to iOS 7.1.2 which is what you have in your signature

You realize, you do NOT get points simply for marking your own reply as solving the issue, right?

Also, the answer remains the same, no matter how many times you ask.

Your signature indicates your phone is running 7.1.2. That's as far as iPhone 4 can go. If you need iOS 11, you need a newer phone. That's the only choice you have.