How do i become the alpha dog with a 6 year old ...

That is a difficult situation.  You have to be careful so this dog doesn't become violent.  Becoming very good friend with your aggressive dog is a good start.  Although they feel powerful, they don't want their friend to be mad at them.  If you bond with the animal, then slowly begin training with treats... the animal will start to respect you.  It might take a little longer with a 6 year old, but make sure you keep it up. 

if u really want to become the alpha dog u have to stair the other dog down even if it takes hours. then u mount them

Speak to Allan Morris at the canada dog lovers network  He is a canadian dog trainer who answers questions like this on the dog's website chat line every wednesday evening for one hour at www.dogswebsite.ca

For me its about stance and attitude.  Mine not his.  I stand up and keep relaxed but get in that Marine stance(no fear) and say no loudly whenever he doesn't do what he should and then when he stops, I pet him and tell him good boy.  If I have to I back him up a little to let him know he has not dominated me.  Both of my dogs know that I mean business once I stand up in front of them.   Just don't forget the praise when he does well.  You don't want to stand up to a dog that is already fighting or a strange dog though.  That can be very dangerous.

It's Very simple, she testing you against your other dog, if the dog you had first isn't being aggresive enough to hold her or his place, as alpha, she's intiminated by the new dog, their is nothing a human being can do, it's totally up to her to reclaim her postion again, she is insucure, show her a little more affection without making the other dog feeling left out, take an old t-shirt that you wore the day before, rip it in half and give them the t-shirt that ripped or cut infront of them, (DO NOT WASH IT ), both will know who's incharge within 1 week!!!! If they start to scrap it out, just let them and stettle it,NEVER STOP THEM unless one of the dogs is getting hurt, the first dog that rolls on their back, the other dog will have established who the boss is and won't go any further with it!!

hand feed her- don't leave food down all day- walk in front of the dog when going somewhere. Take charge.