You cannot currently move an existing Asset from one Asset Type to another. You will need to create a new Asset under the correct Asset Type and archive the original Asset.
In the Web Portal go to Assets > New Asset > Select the correct Asset Type > Enter the Asset details > Save Changes. Once completed, go to Assets > Asset Register > Select the original Asset > Archive.
Please Note: Archived Assets can be reactivated if you need to use them under their original Asset Type again.