My daughter has my maiden name and me and my husband both agreed to change it to our last name? But we don't know how to go about doing that?

The best solution would be to use an attorney. A cheaper alternative is to use a name change kit. These will insure that you meet all legal requirements in your state. The following links will provide more info and kit sources.

Change Your Name - name change, forms, do it yourself, change your name, how to, change name, change of name | My Family Law

Name Change overview - LegalZoom.com

Is your husband the legal father of the child ? If so you just need to go to court and change it. If not and it is another man then he will have to be contacted for the name change to be approved.