I gave birth to my first daughter unmarried and has my last name. Now I am engaged to the man that has raised my first daughter with me for four years now, but is not her legal father. We also have two other children together and they have the same last name as my fiancé. My first daughter will be starting school soon and I would like to change her name to my fiancé last name so then there will not be any confusion and her and her siblings would have the same last name. I don't know how to go about doing so in Missouri. Also my first daughter's real father is not listed on the birth certificate or NEVER has called or had any contact with her in four years. Can't I say its unknown? But how do I change the last name?

You cannot say it is unknown if it is not. That would be a crime (perjury).

Since he has not had contact and paternity has not been established the court may not make an issue of it. Chances are they are going to require a good faith attempt at contacting him first though. Each state will have their rules as to what constitutes a good faith attempt.

Also, if you can't get her name changed, you can use any name you like for her. Just like a nick name of sorts, but she will go by that last name through school if you really wish for her to. Besides the paperwork, like birth certificates and signing up for insurance or signing up for sports, you may have to use her real last name . You can always make it clear to her teachers and coaches that you would like to have the other name used. For instance the kids would know her by her step dads last name, and she can use her step dads last name on a jersey, but in the legal paperwork it would be different. Good luck

A birth certificate is a legal document. To change it requires a court order.

Check with your local family court about the process.