Resetting student attempts on mobile-friendly assignments is done via the statistics page.
Click Grades, then click on the name of an assignment in a column header. Or, if you're already in the assignment, click the Stats button at the upper right.
Navigate to Assignment Statistics
Click Reset Attempts in the upper right corner of the page.
Choose questions to reset
Click the [x] in a cell to reset a single question for a single student. Click the [x] at the far left to reset the whole assignment for a single student. Or, click Reset entire assignment for all students.