As per the Envato Elements license terms, you must relicense an item for each project you use it in - even if you've already downloaded it. In order to license an already downloaded item simply visit your 'My Downloads' page, find the item you want to relicense for a new project and click 'Add a License'.
You are then able to license it for another project (for instance, when the item will form part of a deliverable for a client). To help you keep track of your licenses, you must name each project you are obtaining a new license for.
Not only are you legally required to relicense an item for each individual project you use it in, but doing so means the contributor who created it will be paid accordingly for your usage.