Facebook has a limit to change your birthday to avoid abuse and fraud. If you have changed your date of birth, it will appear in the list and you can choose from there. If you have not changed and can not change at this time, you must wait a moment, it can vary between a day and a month to enable it. If you waited long enough and still can not change, contact Facebook.
From the Facebook Help Center:
To avoid fraud and abuse, there is a limit on the number of times your date of birth can change. If you have recently changed it, you may have to wait a full day before you can edit it again.