To assign and remove forms in libraries, first navigate to the libraries tab in the portal.
Next, click the manage button and click “Forms”.
From this view you will be able to add and delete forms by clicking on the add and remove buttons on each side of the columns. Assigned forms will remain on the left and unassigned users to the right.
When you are happy with the changes made, click the "X" button in the top right corner and your changes will be automatically saved.