Once you set up assignment groups and add weights, you can change the weights via the Gradebook.
Click the Assignments link.
Click the three-dot icon [1].
Click the Assignment Groups Weight link [2].
Click the Weight final grade based on assignment groups checkbox to weight the final grade based on groups.
After selecting the checkbox [1], type in the various weights for the assignment groups [2]. Click the Save button [3] to update the group weights.