This is a painstaking and very cold job as you are certain that your fingers will get frostbitten. And if you have a chest freezer, you have to dive head first to reach the bottom, to remove the ice that has fallen in and the icy water.
Here is a little tip that will make your life easier! After you defrost your freezer, spray or smear a little oil on its walls. The ice that will accumulate again will come off almost by itself the next time you defrost your freezer.