You must be a Group Admin to remove a member, Currently, this option is only available from the Desktop client or Mobile app.
From the Mobile app:
1. Tap on the Group
2. Tap on the ‘3 line’ Menu on the top right
3. Tap on Members from the drop-out
4. Tap on the user you wish to remove.
5. Select Remove.

From the Desktop Client:
1. Select the Group
2. Select the ‘three Dot’ menu on the group window
3. Select ‘View members’ From the drop-down list
4. Hover over the member and select the X to the right of the name.
5. Select Yes to confirm the removal.