From the details of a publication (see 6 of the help on the search), click on the “Change type” button.
Choose the type of publication (see 1) in which you want to convert this publication. The “Before” (see 2) and “After” (see 3) zones allow you to see the changes that will be made during the conversion. Once you are sure of your choice, click on “Continue” (see 4).
Adapt the metadata to match the new type selected. Then click “Save” to save the type and the metadata changes.