It's really easy: Just click on the calendar icon in the personal details form, choose the day you were born on and then click on the month and year at the top of the calendar to choose them, retrospectively.
Here's what it looks like:
If you have entered the wrong date of birth, please let us know via email and we will change it for you. It is important you let us know your correct date of birth, because when we validate your ID, your ID verification will fail if we cannot match the date of birth against your application details.