How to tell my daughter about her biological father who isn’t in the picture

Hello my name is Brandie. I’ve wondered this for years now and I have no idea where to begin, I hope any of you can help me in some kind of direction please and thank you!

My daughter will be 6 this august. I left her abusive father when she was only 2 and a half months old. I met my now fiancé when she was 4 months old and he’s the only dad she knows. We’ve tried visitation before but her biological father has only seen her maybe a couple times and the last time she was 3. How and what is a good age to explain the situation to her that her dad isn’t het biological dad? We have a baby on the way and she’s already wondering why it won’t have her last name (which is mine). I want to do this for her to understand but before she’s a teen. Thank you for any advice sorry for the run on!