How can I stop flickering and ghost touching on iPhone 6+?

if a restart doesn't help, a factory reset might. if still nothing, a visit to Apple Care is a good idea.

The iPhone 6 Plus is going to need a new screen. About $200 at the Genius Bar or authorized Apple service shop if parts are still available for this older phone.

You can find a list of authorized shops in your area using this link: Find Locations

There may be some question about about whether Apple will still support the older phone....and if they do.....another question might be whether or not the money would be better spent on a new phone.

Good luck.