This article covers how to give a collaborator the Upgrader permission on an enterprise account. 

If you are an enterprise administrator you can add an Upgrader permission type from the user management page in the admin panel. Locate the collaborator that you want to give this permission to by typing in their email address, then click the "Add Upgrader" button.

You’ll then see the Upgrader user type added as a label for that collaborator. Once they are set as an upgrader they should be able to upgrade the workspace to an Enterprise level plan through the workspace settings.