I need to update my iPhone 6

Go to Settings > General, then tap Software Update. If you see more than one software update option available, choose the one that you want to install. Tap Install Now. If you see Download and Install instead, tap it to download the update, enter your passcode, then tap Install Now.

[Edited by Moderator]

iOS 12.5.7 is the most recent version of iOS for iPhone 6.

iPhone 6 is not supported on iOS 13 and later.

If you need a newer iOS version, you will need a newer (used, refurb) or new iPhone.