I want to update my iphone 6

There are different ways to ensure the highest iOS that your device will support is installed on your device.

This support article has instructions about updating your iPhone or iPad wirelessly --> Update your iPhone or iPad - Apple Support

You can also use Mac and PC computers to update system software:

- If you're using a Mac with macOS Catalina or later, refer to this article --> Update your iPhone, iPad, or iPod touch with the Finder - Apple Support

- If you're using a Mac with macOS Mojave or earlier or a Windows PC, refer to this article --> Update your iPhone, iPad, or iPod touch with iTunes - Apple Support

When updating, your device will update to the highest version currently available that it will support, or it will just say it is up to date and can't install a higher version. Only iOS 12.5.7 is currently available for an iPhone 6. Apple indicates ( Unable to update my iPhone 6 Plus from 12… - Apple Community ) I need to point out the obvious and say that if you want to run a higher iOS version than your current device will run, that you need to buy a newer device.