3. Select the Elements 18 Catalog if it is within the dialog box.
4. Click Convert.
If you are having problems finding the catalog, there are a number of possibilities: (a) it may have already been converted, possibly when you first launched 2019, in which case you can check the box for Show previously converted catalogs, or (b) your catalog is in a custom location, in which case, click the Find More Catalogs button to open a browse window.
If you don't know where the catalog folder is located, open 2018 and select the Help>System Info menu choice. After a while, a dialog will pop up which includes the path to your current 2018 catalog.