Stand on a ladder so you can easily reach the ceiling tiles. Use two hands to gently push an old ceiling tile up. Tilt the tile so it fits between the grid opening, and lower it out of the way.
Lift a new ceiling tile up to the grid opening. Tilt the tile so it fits through the opening. Straighten the tile, and gently lower it onto the grid. Repeat this step with the remaining ceiling tiles.
Cut new ceiling tiles to fit in smaller grid openings, using a utility knife. Remove the old ceiling tile and lay it on the new ceiling tile. Trace along the old tile's edges lightly, using a utility knife to mark where you need to cut. Remove the old tile and hold a straightedge against the score line on the new tile. Run the utility knife down the straightedge to finish cutting through the new tile, and then lift the tile into the grid opening.