If you want to upgrade to Catalina, best way is to:
1. Create a bootable cloned backup of Mojave using CarbonCopyCloner, which is
free to download and use for 30 days. You want a CLONED BACKUP that is BOOTABLE because if you run into problems, having this makes it easy to "get back". Without one, you can get back, but it WON'T be "easy".
Get CCC here:
Carbon Copy Cloner - Download
2. Boot to INTERNET recovery (NOT to "the recovery partition")
Command-
OPTION-R
at boot
3. Open Disk Utility, go to the view menu and choose "show ALL devices" (you MUST do this to go further)
4. Select "the top item" in Disk Utility's list. This is the internal drive.
ERASE it to APFS, GUID partition format.
5. Close Disk Utility and open the OS installer. Begin "clicking through". Be aware that the Mac will reboot one or more times, and the install will take some time.
6. When done, you should see the setup screen ("choose your language"). At this point I suggest you plug in the cloned backup, and begin clicking through.
7. At the appropriate moment, you will be asked if you wish to migrate from another Mac or drive. YES, you want to do this.
8. "Aim" setup assistant at the cloned backup. GIVE IT SOME TIME to "digest" everything.
9, Setup assistant will present you with a list of all the things you can migrate. I suggest you just leave everything "as is", and "let 'er go". Again, this will take some time.
10. When done, you should see the Catalina login screen. Log in and look around.
HAVING WRITTEN ALL THIS:
If things are going just fine for you right now with Mojave, my best advice is:
Just leave things alone.
Catalina has problem to be very problematic for too many folks right now.
