MillieT03 I can try to make the steps more specific, but I'm not sure which part you need help with. If you open the EA App and click Sims 3 in your library, or in the left panel, and then click Manage, then View properties, you'll see the game's install location. This is the default location, which you could have changed:
C:\Program Files\EA Games\The Sims 3
Write this down if you're not going to remember it.
Close the EA App by clicking the three lines in the upper-left corner of the screen and selecting Exit. Right-click the Task Bar at the bottom of your screen, select the Task Manager, and under background processes, find the EABackgroundService. Right-click its name and select End Task.
Open a File Explorer window and navigate to the install location. Inside the Sims 3 folder, you'll see one folder for each expansion or stuff pack you've installed, along with some others. The EP07 folder corresponds to Supernatural. You can delete it by dragging it to the recycle bin. I think there's a Delta folder with other data inside, but I don't have my Windows install in front of me at the moment (it's at home and I am not), so I can't check. If you don't see one, that's fine, skip that step.
Restart your computer and clear the EA App's cache (linked above). Open the App, click Sims 3 > Manage > Manage add-ons, find Supernatural on the list, and you should be able to download it again.