How Can I Legally Change My Daughters Last Name?

I really wish i was more educated on this before giving two last names to my daughter.

Oh well, here's my question.

When my daughter was born i wasn't married at the time. I was raised up thinking that you only get your father's last name ONLY if your married. It was confusing to decide as what last name to give to my daughter as i really wasn't sure if my spouse was ever going to marry me.

Don't get me wrong he loves me, he is just one of those guys who doesn't believe in marriage and says its just a ring and piece of paper...

At the time i wanted my daughters middle name to be Victoria so she would have ( insert first name here) Victoria (Last name here)

But we weren't married, so i decided to give her both mommy and daddy's last name.

For the sake of conservation lets say the last names are Bob and Christy.

Now my daughter has 4 names ( insert first name here) Victoria Bob-Christy. This was decided before marriage.

2 years later after knocking some sense into the man we are now married I Now have his last name, we now have a son he has his last name. Daughter is 5 years old now. I don't want her to grow up feeling left out with only one having 4 names in the family.

Bottom line I would like to drop the Bob last name and Give her Christy.

So instead of reading as (first name here) Victoria Bob-Christy It will now read as
(First name here) Victoria Christy.

Where would i go about changing it so i can legally drop one of her last names as Christy Only???