You can change the weight type of your gradebook categories
Tap on 'Configure Gradebook' (top right), to start configuring your gradebook.
The below dialog will be opened to you (first category is selected by default):
Tap on the category name (the middle title in the topbar), to open the categories view.
It lists all the categories in your gradebook.
Just tap on "Edit" to go to the edit mode (Premium Use).
Tap on any of the categories to to edit its details - including its weight type.