Decide If You’re Upgrading or Doing a Clean macOS Install
There are two ways to install macOS Catalina on your Mac. You can update and keep everything in place, or you can do a clean install that gives you a fresh start.
The update option is the easiest and the one most users will want to take. This keeps all your files, apps and photos in place and takes the least amount of time.
One downside is that it can bring along problems and it keeps all the junk that you haven't cleaned up. If you don't manually remove stuff a clean install can wipe your drive clean and gain back some space as you only install the apps and download the files you need.
You can do a clean macOS Catalina installation using this guide, just make sure that you took the time to back up all your important files since they will be gone if you don't have a backup.