If you are considering upgrading an off-domain computer to Catalina, you should check the compatibility of any additional software you have installed. Please refer to Apple’s support article How to find 32-bit apps on your Mac.
If all of your applications are compatible, you may proceed with the Catalina update. You can upgrade to Cataline by doing the following:
Apple Menu > System Preferences > Software Update.
As with all OS updates, we recommend that you back up your files before updating.