How do I stop ice from forming on floor of my Maytag upright freezer?

I suspect that your door seal is leaking in air. The moist room air condenses and freezes. Close the door with a strip of paper crossing the seal. Slide the paper up and down back and forth to see if it meets resistance. There should be some resistance all around the door edge.

Check for a drain hole and clean it. Frost free freezers go through a defrost cycle turning the frost to water which runs down through a drain to a pan under the freezer.