Can't use Facebook on my iPhone 6

There is nothing you can do. Facebook requires a higher operating system than your phone is capable of running.

If you want to use the Facebook app, you will need to purchase a newer phone capable of running a higher iOS.

You can try using the web version of Facebook.

Hello ~ You cannot update your iPhone any further and you need a newer device. If you consider a second hand device please take a moment to read these helpful links by clicking on below…

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~