Open the sidebar, click on Training Assignment and choose Assignment.

Enter the name of the course in the search bar. The results are shown.

Select the required course by clicking on the circle under Select, for the required course.

Now click on Enroll Groups, a grey box in the upper right corner of the screen. The Course Assignment - Enroll Groups screen opens.

Enter the name of the group in the search bar. The results are shown.

The assigned groups have the checkboxes, under Select, checked. Uncheck this box to unassign the group. 

Using steps 5 and 6, you can unassign multiple groups.

After unassigning the groups, click on Confirm, a blue box in the bottom right corner of the screen, to save the changes.