How to get our first ID user apple

about the apple update. I changed my ID user apple because i got a new gmail adress. Yesterday, i tried all my update in the app store. Since the osX update, i'm not able anymore to open all my documents words, and excel. App store ask me my ID user and says that this ID user is not authorized to load these update. They ask me to find my first ID apple when I first made my inscription when I bought my mac book pro. I really need these update. I juste made OS X captain and now, the program numbers need update to resolve problems with excel. BUT I Cannot..... I don't remember my first ID how can I just load my last update