You can deactivate a license by clicking 'Deactivate' on the help tab before you change computer, or before you format.
If you don't have access to the application to do this, simply log in to the user management page at https://go.chimprewriter.com/account-login . Go to the software tab, and you will find your current activations as well as a link to deactivate them. The last activation date is also listed, and your best bet is to deactivate the oldest one. Then you should be able to register on the new PC.
Bear in mind that you can only deactivate once every hour.
If these options didn't work, https://chimprewriter.freshdesk.com/support/tickets/new and we will look into it further!
These instructions also apply to transferring a license to a new computer.