Login to your web dashboard. Under Customers, find Enrolled Customers tab - search by name or email address, tap on edit - enter birth date. If you would like to mark this customer as a favorite then click on the check box. If you have a birthday offer setup these customers will automatically qualify for a birthday reward. 

In addition, if you have a birthday offer setup - every offer that goes out from you to those customers who do not have birthday information yet will see a link at the bottom of the offer to sign up with the birth day and month to receive special birthday rewards. If the customers updates the date and month, the customer record gets updated automatically.