Screws used to fix in the screen no longer usable

The plastic frame around the screen is where the 2 threaded holes are for the bottom screws. if the pieces have been broken off the only way to properly fix it is to either separate the frame from the glass and replace that part (this is not an easy option). Your other choice is to replace the screen with a new assembly. This is an easy option and should not be too costly for an iphone 6 since there are lots of options for screens. You can even buy the parts from ifixit if your concerned about buying from a reputable seller but you can also buy from other sources just do some research on the seller and the quality of their parts. The last and probably the cheapest option is to simply use some adhesive like tesa tape or similar to attach the screen. This is not a very secure solution but if you just want a cheap fix it will work. There are a lot of different adhesives available but the phone would still be able to separate the screen easier than it would with a new screen.

You would also need to buy a pair of new screws for the bottom and the screwdriver for them if you replaced the screen assembly.