You'll first want to find their registration under "Search and Manage" under the "Registrations" tab.
Once you've found who you're looking for select their registration.
On the right side of the screen under Registration Details, you'll find the Team Name header. The Team Name will be displayed and be click-able to the Team Details page. Click the Change button beneath the team name which will open the Team Change overlay window.
In the Team Change window click the red button Remove from Team.
Now that you've removed the participant from the team, you'll notice the Team Name and Team Category will have changed to N/A.
IMPORTANT REGISTRATION CATEGORY NOTE: Some events have team-specific registration categories. Once you remove the participant from the team be sure to review the Registration Category as you may need to change it so there is no conflict. The system will not automatically change the registration category.