Cleaning a fish tank in your wall is basically the same as cleaning a tank outside your wall. The only difference is that you have less elbow room. First you will have to gain access to the top of the tank. This is typically achieved through the top piece of trim. The trim flips up on invisible hinges or lefts off with Velcro etc… If the backside of the tank is not finished you can leave it open at the top and access it that way.
Once you have access maintenance proceeds like any other tank. You will remove around 1/3 the water through a siphon tube. While you are sucking the water out the waste in the gravel should be removed. When the correct amount of water removed replace it with tap water and some water conditioner.
The tank does not move and the fish do not come out.