There may be various reasons why you need to "unenroll" someone, such as:
A Customer accidentally enrolls someone in the wrong Program
A Customer accidentally enrolls the same person multiple times
An Admin user accidentally enrolls someone in the wrong Program
Here's how you unenroll someone:
Step 1 - Move Enrollee from the incorrect Program to the correct one. Learn how to do that in this support article.
Step 2 - Enroll the Enrollee again in the correct Program. This article shows how an Admin can do this. Then delete the incorrect Enrollment. Learn how here.
Step 3 - Delete the Enrollee record. Find out what happens if you delete an Enrollee. You will only want to do this if no transactions have been made for the Enrollee, and you don't need to keep any history.