If you install PSE2021 on the same computer, the installer will search your computer for your current PSE6 catalog and prompt you to 'convert' it to the new database format. PSE6 is compatible with the converter.
This will create a new catalog in the current format with practically all your organization apart from a few features like slideshows or support of acrobat pdf files in your catalogs. The PSE6 catalog will be renamed with a suffix " 1" and still work with PSE6.
If you have several catalogs, you'll have to start a new conversion for each of them in the catalog manager.
Don't hurry to delete PSE6, it's perfectly possible to keep it at the same time as the PSE2021 version.