First head to your Creator Dashboard, you can get there by selecting the link in your user menu.
Once on your Creator Dashboard click on the Manage Products tab and then click the Edit button next to the product that you want to edit.
Once in the product edit screen click on the 3. Demo Video, Categories, & Tags bar, you'll find the tags section for the product at the bottom of the screen, add in your tags after each tag either add a comma or hit the return key to add the tag to the product. Once you're done make sure to click the update button to save your changes.