If you lift up the arm and look at the splined part it goes on to, you will see a small metal tang under there. You really have to look almost from underneath it..its hard to fit your melon in there to see.
But its there. get a small pocket screwdriver and pry out that tang while pulling up on the end thats attached..
You may have to pry lightly with the screwdriver on the arm itself to get it started.