Students get one sitting of each chapter exam, then it closes. Only you can give them another.
Open the student's record
From By student, click their name. Their exams are listed by chapter.
Choose Reopen on the exam
It's next to Adjust on any exam they've taken.
Pick who gets another try
Just that student, or Everyone in this class. You can add a reason; the student sees it.
They're notified, and the exam opens for one more attempt. Their earlier attempt stays on record. Reopening never shows them the answers, but if you have already released this exam's answers, the retake is open-book. See Release a chapter exam's answers.
A question was wrong for everyone
Open the exam paper under its row and switch to How the class did. On any question, Credit to the class gives the mark back to everyone who missed it. If the question itself is wrong, please tell us so we can fix the book.