Depending on how thick the old sealer is, a good
DECK STRIPPER, would do the job. Follow up with a powerwash, [if experienced], or a scrub [with the grain] and rinse. Last but not least, apply a
WOOD BRIGHTNER, with a final rinse, and let it dry for 24-48 hours depending on weather before sealing. One great thing about hardwood decking is that it is far more forgiving than softwoods such as cedar.