1. Disable Steam cloud for Runbow.
2. Create a new folder called "Runbow Files." Inside that folder, create two new folders, "Main" and "Runs."
3. Navigate to AppData\LocalLow\13AM Games
4. Cut the RUNBOW folder to the "Main" folder
5. Start Runbow -- it should load with default settings 5.5 - Set your settings if needed
6. Close Runbow and copy the newly created RUNBOW folder to the "Runs" folder

You are now able to do a run. To reset, delete the contents in the 13AM Games folder and copy the RUNBOW folder from "Runs" into it. If you have no custom settings, all you need to do is delete the contents in the 13AM Games folder every time you reset.

At the time of writing, this is the best way to reset on PC. It also lets you play as other unlockable characters in adventure mode.