I replace all substrate at least once a month, and spot clean when I see the need. Your UTH should be under the glass, unless you are submerging it in a bathtub, it should not be a problem. I just dump everything out of mine and make a mild bleach solution and wipe it down good with a rag, then use clean water and wipe it several times until I cant smell the bleach anymore.

You might not be able to see the liquid urine, but you will see the urates, then I just grab all of the damp cypress around the urates and replace with fresh. I'm sure everyone does it different, but that has work for me so far.