You cannot change a birth date unless the date on your birth certificate was recorded in error. In that case, it would be legal to correct the mistake by supplying the required documentation.
The short answer is no, you cannot change your birth date. You were born when you were born, and this date is recorded on your birth certificate in order to prove your identity. Rewriting the official records to change your birth date could be seen as an act of fraud. The only exception is if the birth date was recorded incorrectly. In that case, it would be legal to correct the mistake by supplying the required documentation.
TL;DR (Too Long; Didn't Read)
You cannot change a birth date unless the date on your birth certificate was recorded in error.
Learn Your State's Rules
States have their own rules for correcting birth certificates. Some states will change your birth certificate upon the presentation of an affidavit with evidence of the correct birth date. Others require a court order. To find your state's rules, navigate to the Center for Disease Control's Vital Records website. Click on your state's link, then read the section titled "Birth." This gives you the address and contact number for your state Department of Health Services, which you can contact for information.
Correcting a Minor's Birth Certificate
Most states will correct a birth certificate within one year of a child's birth without requiring a court order. While each state has different requirements, there are usually a few forms required to make the change. For older children, parents typically must provide evidence of the correct birth date and complete a simple affidavit. Evidence is generally acceptable if it was established close to the child's birth and comes from an official source such as hospital records, a pediatrician's record, immunization records or a baptismal certificate.
Correcting an Adult's Birth Certificate
To correct the month, day or year of an adult's birth certificate, submit an affidavit to your state's vital records department together with evidence of your date of birth. This might be a census record or an elementary school record showing the year you entered school. The record you submit must show the full date of birth and not merely your age. The state department of vital records can help you work through the application and tell you what evidence you need.
Getting a Court Order
Some states require a court order before they will correct a birth date. You can file a petition for correction of a clerical error with the family court in the county where the birth was registered. Attach evidence of the correct birth date as set out in the documents described above. The judge will either approve the correction or invite you to attend a hearing so she can hear your case. Once you receive a correction order, file it with the Vital Statistics office to have your birth certificate changed.
Informally Changing Your Birth Date
While you have to record your legal birth date on your birth certificate, there is nothing to stop you from celebrating your birthday on a different day. People have been known to lie about their age. As long as you write the correct birth date on official documents, you can have any birthday you choose.