While in a class, you can perform bulk actions on multiple assignments instead of having to change their settings individually.
To start, go into the class you need:
Tap 'Edit'
In this mode, you can select multiple assignments by tapping on the check boxes
Select the action you'd like to complete
Depending on the assignments you have selected, you'll get the option to change the student access level to locked, view-only, or editable. You can move them into folders or archive/restore them.