wants to deactivate the purchase code on the currently active domain
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 theme license.
If you don’t find options in wp-admin then you have to contact theme author and request them to unregister/deactivate the theme license for 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.