This report outlines the necessary steps, prerequisites, and important warnings regarding the upgrade from macOS High Sierra (10.13.6) to macOS Catalina (10.15).
The upgrade process (typical path)
If your Mac is running slowly on High Sierra, a clean install is better than an upgrade. all files to an external drive. Boot into Recovery Mode (Command + R during restart). Use Disk Utility to erase your Macintosh HD.