Click the "Deactivate" button next to the seat you want to deactivate 
After you are done you can activate the tool on your new machine.
If you have only bought a single license, you don't need to use the License Manager each time you switch machines. Simply activate the tool on your new machine and it will automatically deactivate the old one.
What are LemonSqueezy and Gumroad?
LemonSqueezy and Gumroad are both payment-processors used by software-developers to handle payments and manage licenses.
You may have heard of equivilant services such as Ko-fi, Patreon etc.