Click "Checklist"  activity in the course.

If you have optional items in the checklist you can choose to hide the by clicking "Hide optional items" button.

You can quickly see the progress bar of each student by clicking "Show progress bar" button.

Progress bar indicates how much of the activities are completed.

Go back to editing by clicking "Show full details".

Click "Toggle Row" to mark all activities as completed or not completed for that student. Keep clicking the button to change the choice.

Alternatively, select individual activities to mark as completed or not completed.

If the activity is already done by student, it will have the green tick next to the dropdown.

You can see which activities you marked as completed or not.

Edit Activity Progress in Moodle Checklist