Go to the SPA ON AIR sauna in T1, B1 basement 1 floor of the public area (after you ex)it baggage/customs of course - but no private bathrooms, will very much be a shared thing

Or you book the DARAKYU CAPSULE HOTEL in the train terminal building

that being said, it's a lot of work and I would just wait until you get to your hotel, check in and shower - much easier