You can buy some Urine Remover from Clorox which is known as Clorox Urine Remover and you can then spray the mattress with the Clorox Urine Remover and then let it sit on the mattress for awhile.
Then use an upholstery cleaner to clean the pee out of the mattress.
You can also spray the mattress with some vinegar and baking soda and then use the upholstery cleaner again to deep clean the urine out of the mattress.
Hydrogen peroxide and Baking Soda also works at removing urine smell out of a mattress or other furniture as well and you can even use some essential oils mixed in if you want too.
After cleaning the urine out of the mattress I would recommend putting an absorbent mattress topper on the mattress that you can remove and wash in the washing machine if the urine gets on the mattress protector again.
You can also mix in a spray bottle 8 ounces of hydrogen peroxide, 3 tablespoons of baking soda (add with a funnel. , and a drop of liquid hand dishwashing soap.
Then spray that on your mattress and it will remove the urine stains as well as the pee smell but you also need to use an upholstery cleaner to help lift out the urine from the mattress to make it the cleanest.