Click on the name of the group from the Groups area of the LMS.
You will be taken to the Explicitly Added Users tab, where you will see a list of any Users who were added to the group explicitly. Click the next to a User's name to remove them from the User Group. A popup will open - click OK to confirm the removal.
Click on the Selection Group tab to remove any Organisations, Organisation Units, System Roles or Categories which have been added to the User Group.
Again, click the next to the selection group you wish to remove and click OK in the popup window to confirm the removal.