If it is too far gone, nothing will help, but if savable, this will do it.
https://colorplus.com/products/soffener/
Seats in my turbo Bimmer are 30 years old. I have used this on them, along with the Surflex colorant. They are due for another treatment. Granted, they are water buffalo hides, which is more durable than cowhide, but for their age, look pretty good.
Stuff is expensive. If you are wanting to go the economy route, I have had decent luck with mink oil. But... it will smell up the car for a few days.