uninstalling O/S High Sierra 10.13.6 Mid 2012 and installing Catalina on the same MBP

❗️ Do NOT upgrade

…until you have made a Time Machine or other backup of your startup drive.

Learn How to Back up your Mac with Time Machine

Uninstalling the current version of macOS is not necessary. Upgrading the OS will replace that with the newer during the process.

Please see this for guidance:

Update macOS on Mac - Apple Support

You should find the upgrade in System Preferences > Software Update.

If the guidance in the link above doesn't help, please see this one:

How to download and install macOS - Apple Support