You can edit marks for students in a previous class by going to:

Menu > Academics > Examination > Exam Management > Create Exam

1. Select a year.
2. On the class-wise tab, select the Inactive classes radio button.
3. Click Manage exams beside the class you want.
4. Click Manage exams beside the exam group.
5. Select a subject, and then edit the marks.
6. Select the Retake check-box if the student is retaking the exam.
7. Click the Save button.