Sorry for the n00bish question in advance:

I want to install an updated version of a theme, with identical file names to a theme I have currently installed. I can't install both because it says the files are currently in use and therefore cannot be overwritten, but I can't figure out how to remove the files. Thanks in advance.

If it helps in any way, I am trying to go from Ultimum 1.2 to 1.3.

1: Switch to a different theme
2: Delete the 1.2 version
3: Goto your Popup select litestep/Change Themes/Refresh list
4: Install 1.3
5: Email the Author and have him release his themes with version #'s

ex: Folder names
Ultimatum-1.2
Ultimatum-1.3

If you release mulitple versions you should release your themes this way... It gives users the ability to go back to another version also!

Or you could always just extract the theme manually into the themes folder into the Ultimatum-1.3 folder and then click refresh list as above. Then you can have both versions.

I would do that, but I can't figure out how to delete the theme, even when I'm using a different one.

delete the folder, delete its entry in themeslist.rc