HOW DO I CHANGE THE LAST NAME ON A BIRTH CERTIFICATE IN FLORIDA?

AN EX-GIRLFRIEND JUST CALLED TO INFORM ME THAT HER BOYFRIEND IS NOT THE FATHER OF HER BABY. AFTER DNA TEST, I FOUND OUT THAT I AM. HOW DO I GO ABOUT CHANGING THE BABY'S LAST NAME FROM THE BOYFRIENDS TO MINE OR THE EX-GIRLFRIEND? NEITHER ONE OF US WANT TO GO TO COURT AND DO THE WHOLE CHILD SUPPORT THING. WE TRUST THINGS WILL ACT MATURELY BETWEEN US.