If you have not already updated your Mac OS to Catalina please backup Garmin BaseCamp first (so you can transfer your existing data)
1. Backup Garmin BaseCamp - more here
If you updated to Catalina on your Mac and found Garmin BaseCamp is no longer working -
2. Uninstall BaseCamp on your Mac - more here
2. Download Garmin BaseCamp again and install a clean version - more here
I hope this helps.