If you change a question or its mark scheme after students have answered, you can remark the work that was already submitted rather than adjusting marks by hand.
You can also change a mark scheme while you are moderating, preview it on one student and remark from there. See How to update mark schemes and remark submissions. This article covers the other route: you changed the question, and now the class needs catching up.
Change the question first
Open the lesson from My Library and make your change: the marks, the correct answer, or the mark scheme. Save it. You will see a note that the question already has submissions.
Then remark
Open the classroom from the sidebar, go to Assignments, click the assignment and then the lesson. On the Students tab, a banner says Some submissions were marked using an older version of the question. and names the questions that changed. The question's column in the table is flagged too.
Click Re-mark affected questions, check which questions are listed, and click Remark.
Remarking runs in the background. The banner switches to Remarking in progress and counts the answers as they are remarked, so you can see how far it has got. A big class can take a few minutes.
Answers you have marked or adjusted yourself are left as they are. A lesson assigned to more than one class shows the banner in each class, and each class is remarked separately.
What triggers a remark
Changing the marks allocated to a question.
Changing which option is correct, or the correct pairs or order.
Changing the mark scheme on a long answer or free response question.
Fixing a typo in the wording does not flag anything, so you can tidy up without prompting a remark for the whole class.