Go to Register > Manage Register. Choose the class or individual student in question from the drop down box and enter the appropriate date range. If the student is off roll, remember to select Show Former Students. Then click on View Register.
Click on the incorrect code and from the drop down box choose the correct code for that session. Repeat the process for any additional incorrect codes on that register. You can remove codes or select a new code for a number of days at the base of the page if this is quicker, ensuring the individual student or students are selected in the right column. If only one student requires a code change, make sure other students in your view are not selected in the final column.
In the example below an individual student is selected along with a defined date range and pm only. The present codes for both Tuesday and Wednesday PM can then easily be changed to another code.