A tiled ceiling can be highly attractive. . . unless even one tile is missing; then, it looks terrible. Tiles do often crack or break over time, but they can be replaced with the right kind of attention and preparation.
Step 1—Remove the Old Tile
If the ceiling tile you are replacing was glued rather than suspended, use a very sharp utility knife such as the Exacto to cut very carefully along the four sides along the joint line so that the good tiles surrounding the perimeter don't get damaged in the removing process. Plastic and Polystyrene ceiling tiles are easy to break, especially old ones. If the defective tile is well glued and difficult to remove, break up a piece in the middle of it and keep removing pieces of it working your way to the edges.
Step 2—Clear the Area
Scrape all the residual tiling and dried glue from the ceiling with a scrapper. Sometimes old adhesive can get so hard it is very hard to remove. If you encounter this problem, use a rubber mallet with the scraper to loosen it. The ceiling needs to be totally clean and flat so that no bumps show through the new tile or prevent it from sticking.
Step 3—Dry Run
Place the new tile in the gap and make sure it is an exact fit. If you don't have any leftovers from when they were installed (look around the attic), you'll have to buy some.
Step 4—Fitting the New Tile
If the tile slips nicely into place, bring it back down and place a liberal amount of adhesive on the underside. Spread it around without making the layer too thick. You want the tile to be level with the others around it and not stand up because the glue is too thick. The best way to tell which method to use is actually to follow the pattern leftover from the previous tile.
Once the new tile is in place, make sure there is no residual glue slipping over the edges. That glue will turn yellow over time, creating marks around the edge of the tile.