Login to Altium Designer to access the license management page and see licenses associated with your organization. To do this, select the portrait icon in the top right and select Sign-in.
From here, sign-in with your AltiumLive credentials.
After signing in, click on your avatar icon portrait (Current User Control) button in the upper right corner, to select Licenses... from the portrait icon to access the License Management page. Expand the license you are trying to release by clicking on the small triangle at the left end, to see the list of users currently using the license. Right-click on the user you want to release, to select Release. This would force the user to release the license seat. Any AltiumLive user is able to release a seat they are using (on another computer for example), and a member of the Group Administrators group is able to release any license seat (for any user.)
A group administrator for your organization can also use the dashboard to release a license seat, by going to the reports tab to view the REAL TIME USAGE report, where a seat can be released after expanding the license entry (click the + sign at the left), by clicking the X at the right side of the entry for the user with the seat to be released. Here's a link to our documentation for more detail and images: https://www.altium.com/documentation/altium-dashboard/reports#!real-time-usage