My question involves name change laws in the State of: Texas.

My question is simply how, or can I, change my girlfriends daughter's last name? She has no father on her birth certificate, and doesn't know her biological father. She calls me dad, and has for her entire life. Without getting married is there a way that we can legally change her name?

I would like to do this before marriage, but if not possible I would also like to hear about the step-parent adoption option.

Thanks