My fiancé has a little girl. When she was born the mother told him that she had to use her last name on the birth certificate or else they would not use her parents insurance. Now, that he knows she was lying is there any way that he can change her last name to his if the mother is disagreeing?

Nope. A name change requires the permission of BOTH legal parents.

There is a way of doing it. I am asking how?

If you legally adopt her then it is easy. This however is near impossible if the other parent has even partial custody.

There is a way of doing it. I am asking how?

The way to do it is hire a very expensive lawyer and go to court and prove beyond a shadow of a doubt that the name change is in the best interest of the child.

Or in other words, its not going to happen. A name is just a name and it is VERY unlikely to be proven that it is in the best interest of the child.