Set a ladder in the room under the ceiling panels you want to change out.
Climb up the ladder, then push up on one of the old tiles and turn it perpendicular to the ceiling.
Angle the old panel and pull it out of the ceiling grid. Climb down the ladder and set the old panel aside. Repeat this process as necessary until all the old panels have been removed from the ceiling.
Pick up a replacement ceiling panel and climb the ladder.
Turn it perpendicular to the ceiling, then fit it through the opening and turn it parallel with the ceiling. Gently let the replacement panel drop into place. Insert new panels as needed until each ceiling panel has been replaced.