Sometimes students accidentally submit their work before they are ready for it to be marked. This is the advice you can give them to allow them to save work but not submit as well as instructions on what to do if you want to "cancel" an incorrect submission.
To save their uploaded work but NOT submit they should only use the [Save Changes] button displayed below their submission. This option appears as soon as they upload / enter information for an Assignment.
Once they have pressed [Save Changes] they will see a summary of their attempt together with options to continue editing or to submit, similar to below:
Pressing [Edit submission] puts them back to the screen to enter information and/or alter file submissions.
Pressing [Submit assignment] will display the Submission statement shown below, which does clearly indicate that continuing will prevent them from doing any more work but which some students still accidentally do.
If you have a student that has accidentally submitted an assignment and you are prepared to allow them to continue working regardless of them having submitted (e.g. allow them to do more work without using up one of their attempts) you need to follow these steps.
In the below example Astro Boy's submission which was accidentally submitted for grading is being reverted back to draft status where he can continue to work on his assignment.