We're working on updating the manual.  Go to twamp.tips/files and download the update for your operating system.  Open the file and run the update program, if a warning message pops up, click more info, then proceed anyway.  Follow the instructions on the screen to first run the firmware update, then the effects portion.  In effects, you will see a few options, namely replace overdrive with chorus, and restore to factory settings (this will re-exchange chorus for overdrive).