To get urine smell out of a mattress go to a pet store or Walmart and get some Pet Urine odor neutralizer that comes in gallon jugs.
The pet urine odor neutralizer works for both pet urine and human urine smells as well and then put the urine odor neutralizer in an upholstery cleaner and go over the mattress with the upholstery cleaner.
You can also spray some urine odor neutralizer onto the mattress and let it sit for 30 minutes to an hour before using the upholstery cleaner.
Another option to get urine or pee smell out of a mattress is to take some vinegar at full strength and spray it on the mattress and then pour some baking soda onto the mattress and use a scrub brush to scrub the mattress clean and then wipe it up with paper towels.
After doing that go over the mattress again with vinegar and water in the upholstery cleaner.