Can I uninstall one website license that is already installed on the website that I have?
theme registration maintain through theme author own database. So, if you need to unregister/deactivate then you can do it from your wp-admin. In your pruchased theme options should have options for both register and unregister. If you don’t have access of old website wp-admin where the theme registered/activated then you have to contact theme author and request them to unregister/deactivate the theme license from the old domian/website.
contact theme Author and let them know How to Contact your purchased theme Author, theme author will be happy to assist you.