The Unsubmit button serves to return the Quiz/Question to students in its current state. This can be used in situations where students accidentally submit an incomplete Quiz or if students need to improve their answers.
You can do so from a student's Quiz view by clicking the name of a student and navigate to the Quiz Completion page. Click Unsubmit.
You will be able to Unsubmit the quiz for multiple students from the Heatmap by viewing the Activity with the Teacher-Marked Quiz and selecting the checkbox next to the student's name.

Unsubmit Teacher-Marked Quizzes

To unsubmit a particular question, select the name of a student from the Heatmap and navigate to the specific question in the student's assignment. Click Unsubmit.

Note:

When you unsubmit a Teacher-Marked Quiz/question, feedback and annotations are retained for students to review.
Students will receive a notification when a Teacher-Marked Quiz/question has been unsubmitted.
Teachers can only unsubmit Free-Response Question (not enabled for Feedback Assistant - Mathematics), Audio-Response Question (not enabled for Speech Evaluation) and Multi-Part Questions with at least 1 open-ended question (not enabled for Feedback Assistant - Mathematics and Speech Evaluation).
You will be able to see the name of the Teacher and the time it was unsubmitted.

(B2,1) Release Teacher Marked Quizzes

(C1,i) Add Feedback Assistant Mathematics (Enhanced)