Open SW and de-activate the license.
You can do as Frederick Law suggested...Open/Launch Solidworks and go to "Help", "Deactivate Licenses"
This would be the method I would also recommend.
Or you can just uninstall the software. It should prompt you regarding the license and allow you to tell it to remove the license from that computer.
And yes...the time between deactivation of one machine and activation on another machine does count as part of the 1-year use.
Or you can go to my.solidworks.com, login and go to admin portal. There you may change licensing from machine based (like you have) to online based. Then using your credentials you may simply log in to solidworks app during start. You may install solidworks on as many machines you want.