How To Update Macos High Sierra 10.13.6 To 10.15 [TRUSTED]

Since you are jumping a full version (from 10.13 to 10.15), this is a significant update. Use Time Machine or an external hard drive to back up your files. If anything goes wrong during installation, you can restore your Mac to its previous state.

Ensure you have all 10.13.6 supplementary updates: how to update macos high sierra 10.13.6 to 10.15