Loose, bulging. If tile is applied to wood furring strips, nail back to the strips with flat-headed nails. If applied on plaster, use cement-coated nails. Countersink the nail heads and cover with spackle.
Stained by a leak. Stop the leak. Tile can be painted with any interior paint without damaging its acoustical qualities. But if you prefer to replace the tile, cut it out with a knife. Trim off flanges on new tile as necessary so that it can be fitted into the hole (most acoustical tiles have interlocking edges). Nail tile in place or glue it down with acoustical tile mastic.