Start by clicking "edit" on the discussion or assignment you would like to update. At the bottom right of the screen click on the "Evaluation & Feedback" options:

To remove the rubric, click on the "X" 

Next click on "Add Rubric" and then "Add Existing"

Check the box next to the correct rubric, then click the blue "Add Selected" button

Finally, click on the blue "Save and Close" button at the bottom of the page.