To expand upon @eviltrooper’s suggestion:
Pull the SD card out of your Raspberry Pi. Using an adapter, plug the card into another computer (most likely a Windows-based computer). Once the card is plugged in, the data on the drive will appear. You need to copy the config.js file from the SD card to your other computer.
If you’re using a Windows PC, the computer will display two connected drives. One of them will demand that you format before you can use it. Don’t do that. The other drive will show you a file structure.
On the drive with folders, navigate to Home -> pi -> Magic Mirror -> config. You’ll find the config.js file in there. Copy it to your PC, somewhere where you can find it again.
At that point, you’ll want to restart the process to format the SD card and install the Raspberry Pi OS from the tutorial.