Wall heater is fixed to a wall through clips that provides support to the heater. But when you need to remove the appliance you have to take out the clips as well as the fasteners which are intact with it. This process has to be done carefully as it might even result in injuries that can be harmful. You may also require a guidance that will be helpful in removing it with care.
You must turn off the button of the heater for security reasons and also the main circuit of electricity. Anything can happen so try to avoid such situations. The gas line has to be switched off from the heater itself but it can vary from device to device. Then take a screwdriver and unscrew all the clips and put down the heater. The number of screws depends upon the size of the unit supported. Now you can either get it repaired or install a new one as the case may be.