I want to update my iPhone 6

Hello ~ You can update to iOS 12.5.7 but no further. Click on the links below….

Update your iPhone or iPad - Apple Support

If your iPhone or iPad won't update - Apple Support

If you need a higher version of iOS you will need a newer iPhone. Click below to shop for a new device…

iPhone - Apple Official Site

If you purchase a second hand device please take a moment to read these helpful links by clicking on below to make an informed decision…

Activation Lock for iPhone, iPad, and iPod touch - Apple Support

Also:

The All Too Common SAD Reality of Buying a Used iPad/iPhone

~Katana-San~

An iPhone 5s or iPhone 6 isn't capable of 13 or above; they require at least 2GB of RAM. To install 12.5.7, use a computer or the Software Update function in Settings.

(242204)

iPhone 6 is from 2014. Consider getting a newer model if you want more modern software options, presuming you are looking to install apps that require a later iOS version.