Uninstalling Mix It Up consists of simply deleting the folder wherever you have it. If you downloaded the zip file version, it's just deleting the unzipped folder. If you ran the installer, all that does is just download the same zip and put it into your local app data; there's nothing actually "installed" like a regular program. You can get to your local app data folder by either:
Opening Mix It Up and heading to Settings -> Advanced -> Installation Folder
Opening Windows Explorer and typing the following into the address bar at the top: %localappdata%/MixItUp
If you ran the installer, you'll also need to delete the folder on the Start Menu with the shortcut to run the application.
Uninstalling the application, unless instructed in very specific circumstances, generally won't resolve any issues you might be running in to and isn't recommend as a means of trying to solve a problem you're having.
Note also that deleting your installation also means deleting your associated data, as your settings & data only exists on your computer.