My battery was swollen quite badly, lifting the top of the display away from the case. The glass had separated from the plastic bezel. The screen was discolored in a vertical streak on the left side. I needed to replace the battery.
Everything went swimmingly until I had to put the front panel assembly back into position. Opening it up was not especially difficult, it just required being very gentle because, as I said, the plastic had pulled away from the glass. Removing and replacing the brackets and connectors was a cakewalk. The problem was trying to re-assemble the two halves. The top half just wouldn't fit. It turns out that the expanding battery warped the brackets on either side of the front panel assembly such that they would not clip back into the case on one side! So I removed the brackets. I couldn't straighten them for all my trying, partly because I couldn't figure out in exactly which way they were warped.
If you need to, you can remove the brackets. Now my phone has to live inside a case for fear the front panel will fall off, but it does work. The battery was successfully replaced, and the screen is no longer under pressure from below. It's just not attached at the sides! Which turns out to be less worrying than you might think.
Anyway, if you can't get your top back on, try taking off one or both of the brackets. Do it gently so you can replace them if that's not your problem! You just need a phillips head screwdriver.