On Windows, you best option is to simply run the 1.5 or 2.0 installer - it is very quick. For OSX, I am less familiar with the installer, so you may need to uninstall first.Probably not, but is there a way to run 1.5 and 2.0 plugins side-by-side for testing purposes or for easy switching back to 1.5 if I hit a bug in 2.0 and need to get something rendered?
Alternatively (for the more technically proficient user), you can manually put the Octane 2.0 version in C:\Users\[username]\AppData\Roaming\Luxology\Configs\OctaneRenderForModo2 and then rename the OctaneRenderForModo index.cfg to something else (not ending in cfg) when you want to run the 2.0 version instead of the 1.5 version.
Paul