It's not in the game's menu, but you should be able to change it manually by editing the uimappings file located in your user path (it varies based on OS, so launch the game and go into Options > Mods > show mod path).
The uimappings file is in the /Setting folder, rather than /Mods that the game will show you, but it's the same path.
I'm pretty sure the mouse buttons are towards the top, so you should just be able to swap their IDs. I don't have access to the file at the moment since I'm at work, but once I get home I'll take a look and be more specific if you haven't figured it out by then.
(Note: If you're playing 1.095 beta, there's an issue where it seems the game resets the key mappings every time it's launched, so this wouldn't do you any good, but it should work in the current non-beta release, 1.05)
