How do you change your name in allegheny county?

That depends on the reason for the name change and whether the name change is for a minor or an adult. Changing back to your maiden name after divorce, for example, is a relatively simple process, and can be accomplished by completing an Notice of Intention to Resume Prior Name form available from the Adult Section of the Family Division of the Court of Common Pleas. Certain child name changes related to divorce or adoption are accomplished through either the Family Division or the Orphans' Court.
Adult name changes not related to a divorce can be more complicated. Often fingerprinting and a court hearing are required. Generally, you will have to contact the Civil Division of the Court of Common Pleas to obtain instructions specific to your situation.
See the Pennsylvania Name Change related link for specific information regarding different kinds of name change, and the Allegheny County Courts Guide for Court of Common Pleas contact information and online resources.