Simply click to the left of the group title to grab it and then drag and drop :)
If you have many different groups in your board, you can collapse them for a better visibility.
To collapse groups, click on the small dash icon located below the board/search filter.
Once collapsed, your groups will look like this:
Click to the left of the group title to grab and drag/drop to change the order.
Once your groups are sorted, you can click on the same little dash to expand them again.
If you have any questions about how to sort your groups, please feel free to reach out to our customer success team right here. We are available 24/7 and happy to assist :)