If you have a small amount of changes to be made, you can edit Historical (Stored) Grades on the students' Historical Grade page by clicking into the grade.
If you have large numbers of these errors you can re-store grades for everyone (this will overwrite all the stored grades for that particular Store Code in the term-length(s) you are storing for), or you can use the Additional Filter Options area of the Permanently Store Grades page to store for a selection of students or an individual section.