I wanted to update my phone

iOS 12.5.7 is the latest iOS security update for iPhone 6.

Go to Settings -> General -> Software Update -> Download and Install (if you haven't already done that).

iPhone 6 can’t be update to a higher iOS (13 and later). 

Yes, most apps require iOS 13.4 or later.                                

Considering a newer type of iPhone?

Read this before you buy a newer type iPhone.

This explains how to update your phone. If you are having a problem, you have to tell us what is going on.

Update your iPhone or iPad - Apple Support

If you are having a problem, this might help.

Updating iOS: common issues and solutions - Apple Community

EDIT - I didn’t see that your phone is an iPhone 6.