Assignments that are shared with you for your collaboration are not listed in My Assignments or in your folders because you are not the original author.
List the assignments that are shared with you by searching with the group permission as a criterion.
Click Assignments › Search.
For Permission/Groups, select the permission group for the assignment that was shared with you.
Click Search. Assignments with permissions set to the specified permission group are listed.
Click edit for the assignment that you want to change.
Make your changes to the assignment, and then click Save.