Airtable credits (received by, for example, referring others to Airtable or installing the Airtable mobile app) can be applied toward a workspace which is on a paid plan. Please note that you must be an owner of a workspace in order to apply credits toward that workspace.
To apply credits to a workspace, go to the workspace settings page by clicking the dropdown arrow that appears next to the name of the workspace on the homescreen, then clicking the Workspace settings option from the submenu. (If you are not a workspace owner, you will not be able to access the workspace settings page for that workspace.)
In the Plan and billing section of the workspace settings page, there is an option to transfer personal credits to the workspace.
Clicking the blue "transfer" will bring up a window in which you can enter the number of credits you wish to transfer.
You can also apply personal Airtable credits to a workspace from the upgrade checkout screen by clicking the "Apply up to $X in credit" option above the total.