Apple has a Self Service Repair program – but it is intended for individuals who are very skilled in working with complex, miniaturized electronic equipment.
It also doesn't cover phones as old as the iPhone 6. I checked just now, and
- There is no online repair manual for the iPhone 6.
- The Self-Service Repair Store does not stock parts for anything older than the 12-series phones.
So you don't have the skills, tools, repair manual, or OEM parts to repair that phone. On top of that we're talking about an iPhone that came out twelve years ago - an iPhone so old that it cannot run many popular apps.
It's time to replace the phone.