Well, if you've uploaded a file already..
Go to RTB, sign in at the forums, and then go to the content section of the forums where all the Add-ons are, you should see a "Filecp" button under your name now.

When you click it, on the menu on the left, under Your Files, you can "Update Files".
Select that, select update on the far right of whichever add-on you want to update, then follow the process from there.