Position a ladder beneath the flat ceiling panels that you wish to change.
Remove the existing panels from the ceiling gridwork by pressing upward on a corner of a panel, then grasping two of the panel’s edges above the grid. Tilt the panel at an angle and pull the panel through the grid opening to free it from the ceiling. Some slight bending of the panel may be necessary to fit it through the grid opening.
Measure the grid openings using a tape measure, then purchase recessed panels to fit the grid spaces. Extend the tape from one of the upright edges of the square above the grid surface, to the ledge on the other side to take the measurements. Measure the width of the grid ledges that the panel sits on as well, to determine the widths of the recessed panel edges needed along the perimeter of the panels.
Place the new recessed panels into the ceiling gridwork by reversing the removal process. Angle the panels enough to allow you to slip them through the grid openings, then place the panel onto the ledges above the ceiling surface. Set the panels so the recessed edges along the perimeter extend beyond the grid bars to rest within the grid openings.
Measure the partial grid spaces along the edge of the ceiling gridwork, near the walls. Cut full panels to match the partial panel measurements using a utility knife. Place the panel flat on a work surface, with the panel face upward. Mark a line down the cut edge of the panel that's the same distance from the edge as the width of the ledge along the grid rails. Cut along the line with the utility knife, halfway through the panel. Make a second cut horizontally through the cut edge of the panel towards the cut you made on the panel face, until the two cuts meet. Pull the cut strip of panel free, then angle the panel through the grid square. Straighten the tile once it’s above the grid, then place one edge of the tile on a lip of the square. The space where you removed the strip should allow the panel to sit level in the grid square, recreating the recessed lines on the uncut edges of the paneling.