Count the number of damaged tiles to determine how many you'll have to replace.
Remove all of the damaged tiles. If the tile has water damage, look for the source of the leak.
Measure the length and width of the gaps in the frame to be replaced, using a tape measure. If the damaged pieces are in corners or at the end of a run, measure and cut with the band saw.
Replace the damaged panels by simply removing them and lifting the new one into place. Go first at an angle to insert the tile. This will allow the whole tile to fit above the frame. Simply lower it into place. Repeat this with all damaged tiles.