When in the client panel select the desired listing type in the left menu, then click the 'Summary' button for the listing you want to upgrade, this will open a listing summary page with an upgrade form in the bottom with the list of pricing levels you are allowed to upgrade your current product to. Make sure to add the allowed pricing levels into the 'Upgrades' field of the current pricing level via administrator as the form will not appear if no upgrades are available.
You can change the 'Summary' button to read 'Summary / Upgrade' to make this step more obvious for your clients. The variable name is
'datatable.action.summary' => 'Summary',