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 the wrench icon
Tap Manage List
In this mode, you can also select multiple assignments by tapping on the check boxes or you can tap on Select Assignments which will give you the option to select all.
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.