You can't. You will have to replace the stained material.

You may be able to sand down to unstained wood and re-seal.

Thanks for your advice. This is depressing! We'll just have to see how it works out.

If the floor was NOT prefinished, then the cracks should be sealed and the urine will not have gone down between the planks. Rent an orbital sander and test it out - it's not expensive to rent one for the weekend, then you can reseal it if you are successful in getting down past the stains.

"If the floor was NOT prefinished, then the cracks should be sealed and the urine will not have gone down between the planks"

A site finish does not seal the floor longer than the first heating season.

The wood shrinks, and every joint opens slightly.

From my one-time experience in fixing a urine stain on a hardwood floor which was site finished (renter's litterbox leaked and had no mat underneath), it was easy to sand down past the stain and refinish. So I think there is at least a chance that this is easy to fix. I would certainly try the sanding down before ripping it out and replacing it.

You could sand all the way to plywood and never get the stain out. Just sayin' Remember the sandable thickness is only about 3/16 to 1/4 of an inch. At that depth you hit nails/staples in the tongue.

I have the same problem. I just ordered a wood bleach. I'm going to lightly sand my floor in that area, and then apply the wood bleach. I will then put sealer on top. I hope it works. I will let you know.

Follow all the directions on the wood bleach. Some of that stuff is real caustic.