Coat the tiled surface with a thick layer of paint stripper to remove existing sealer. Lay a plastic sheet over the stripper to prevent it from evaporating. Smooth the sheet out, using a clean paint roller. Allow the stripper to sit for 1 day.
Take off the sheeting and scrape off the stripper and dissolved sealer, using a wood scraping tool. Scrub off any remaining residue using a nylon scrub brush.
Rinse the tile with water, using a sponge. Allow the tile to dry before applying new sealer.
Vacuum the tile immediately before applying sealer to remove any dust that may have settled.
Pour tile sealer into a paint tray.
Apply an even, thin coating of sealer to the tiles, using a paint roller. Work in rows so that each new row slightly overlaps the previous row.