how do i erase this mac to return it

Hi beckyrae,

Read: What to do before you sell, give away, or trade in your Mac - Apple Support

GUIDE:

1. Backup all data you want to save.
2. Sign out of your Apple ID in System Preferences > Apple ID/iCloud > Sign Out.
3. Shut down the iMac.
4. Turn on the iMac, and immediately press and hold Option + Command + R until you see a spinning globe.
5. Once you are in macOS Recovery, verify you are connected to WiFi in the top right corner.
6. Go to Disk Utility.
7. Select "View" in the toolbar > "Show All Devices".
8. Select your main hard drive. This will either be "APPLE SSD...", "APPLE HDD...", or "Fusion Drive".
9. Click Erase, and input the following exactly: (If you cannot select erase on the main hard drive, erase the "Container".)
10. Name: Macintosh HD
11. Format: APFS
12. Scheme: GUID Partition Map (if shown)
13. Click Erase
14. Quit Disk Utility
15. Go to the reinstall macOS app.
16. Follow the prompts, and install on Macintosh HD.
17. Wait for it to complete.

Cheers,

Jack