iPhone 5 screen is loose

I would recommend to either get a case that would not allow it to move. Like Dustin said you cannot just buy the glass and replace it that way. It requires very expensive equipment to do that. I would also look up your serial on Apple's website and see if it's under warranty. If it is, then schedule a Genuis Bar appointment at the nearest Apple store to see if they would swap out the part or phone for you.

https://selfsolve.apple.com/agreementWar...

If the phone is within a year you can take it back to apple for an exchange. I'm sure they will give you a refurb but its better than what you are doing now.