How to Tighten Upper Display?

I just bought a PSP GO, and I immediately want to get to not only customizing it, but also repairing it as well. The upper display is loose from the main body.

When I gently pull on the top half of the GO, the left side of the upper display comes up a little bit, by about 1-1.2 mm. I want to know how to tighten this again so that there isn't such a distance between the main body and the display.

Thanks for any and all assistance.