John,
Nice trailer. Do you have any pics of the whole trailer?
Are you referring to removing the tire, or the wheel from the trailer?
If you are talking about removing the tire, just lay the wheel face down on some cardboard and use a couple of large flat screwdrivers.
Begin by going around the inside of the rim slowly prying the tire from the rim.
It will take several passes around the rim to finally get the tire seal loose from the rim.
Just be patient. Don't try to break the seal in the first few passes.
You could bend/scratch the rim if you try to pry too much at one time.
Remember, patience is the key.