If you have hardware or software that isn't compatible with macOS Catalina, you might be able to upgrade to macOS Mojave.
You can upgrade to macOS Mojave from OS X Mountain Lion or later on any of the following Mac models. Your Mac also needs at least 2GB of memory and 12.5GB of available storage space, or up to 18.5GB of storage space when upgrading from OS X Yosemite or earlier.
To find your Mac model, memory, storage space, and macOS version, choose About This Mac from the Apple menu . If your Mac isn't compatible with macOS Mojave, the installer will let you know.
Make a backup
Before installing any upgrade, it’s a good idea to back up your Mac. Time Machine makes it simple, and other backup methods are also available.
It takes time to download and install macOS, so make sure that you have a reliable Internet connection. If you're using a Mac notebook computer, plug it into AC power.
After downloading, the installer opens automatically.
Click Continue and follow the onscreen instructions. You might find it easiest to begin installation in the evening so that it can complete overnight, if needed.
If the installer asks for permission to install a helper tool, enter the administrator name and password that you use to log in to your Mac, then click Add Helper.
Allow installation to complete
Please allow installation to complete without putting your Mac to sleep or closing its lid. Your Mac might restart, show a progress bar, or show a blank screen several times as it installs both macOS and related updates to your Mac firmware.