Given that the box is plastic, you have at least 3 options:
Try a slightly larger screw.
Fill the mounting hole(s) in the box with epoxy, drill a new hole, and re-mount the outlet
Replace the box.
If it were me, I would do number 3.
Obviously, make sure the electricity to the outlet is turned off before doing any work.
Edit 1 :
OP says the box is metal. That kind of rules out option 2.
To replace the box, you have to:
1) Remove enough of the wall around the box so you can work. Bigger is better than smaller 'cause you're going to have patch the wall anyway when your done.
2) Disconnect and remove the outlet.
3) Loosen/remove any nuts that hold the wire clamps in place.
4) Remove the box, pulling the existing cables out as you do so.
5) Decide on the style box you want to use. Probably want a new work type and just nail it to the existing stud, assuming you're removed enough sheetrock to do that.
6) May want to pull the cable into the box before nailing the box, if you can do that.
7) Wire up the new output and mount into the new box.
8) Oh yeah, last but not least - patch and repaint the wall!