Under the Student top tab, click on the button Options > Registration > Register Student
When below window pops up, you don't have to fill in all fields. At least one name usually works. Click 'Next'.
The below will pop up, and you select the right student. You can see the red circled dropdown already says "Activate existing student record", which is what we want. Scroll down and click OK. It will take you to the student record as it was when the student was withdrawn.
Next, it will take you to "Student Registration: Demographics". You can update the information in the fields if you need; otherwise you scroll down and click "Next". Next it will take you to "Student Registration: Enrollment", where you can update their YOG and enrollment status (e.g. Prereg, Active). Then, scroll down and click "Next". It will take you to "Student Registration: Confirmation" where you can click the Finish button at the bottom-right to complete the re-registration.