Check out page 9 of your owners manual: searspartsdirect.com/manual/4lx3o9htbo-000582/kenmore-2539280414-upright-freezer-parts
You'll find the directions and diagrams that should assist you to clean the frost. The drain plug should be on the inside of the freezer on the bottom, as shown in Figure 1.
I noticed your comment on your other question. According to the manual, you should remove the food from the freezer in order to defrost it.
I hope this helps.