You change or remove grouping in the Grouping window. There you can remove levels, change their order, turn off sorting by the levels, or remove everything with Reset.
How to do it
Click Grouping at the top of the table.
Make the change you want:
Change the order: Use the up and down arrows next to the level.
Remove one level: Click the cross next to the level.
Switch column: Remove the level and add a new one with a different column.
Remove all grouping: Click Reset.
Click Close.
What does "Sort by levels" do?
When the switch is on, the groups are sorted by the values in the grouping column. If you turn it off, the order follows the sorting you have chosen in the table.