problems apps updating!

HI, I am having problem to update some of my iPhone and mac's apps.

I have lost many time my iPhone and I update my previous iPhone more than two times and the last one which was Iphone 5 I gave to my partner because my actual mobile network updated my old iPhone 5 for a new iPhone 6.

I have bad luck with my mac as well because i split some water into it and i have to send to repair it to a mac supporter shop. They did a good job and my mac is working very well.

Well the problem is this when I try to update my apps in my Iphone 6 the system asked me for my girlfriend's Apple ID password and in my mac book asked me for an Apple ID password that I don't know.

So my question is: How can I change this for my own one to update my apps successfully?

I hope your answer support.

Thanks

Javier