Please Help. How do I update my games on Iphone? It said I have someone else's account.?

[Jeffrey]

I used to update my games on the app store perfectly but know when I made my own account, the old account is still there and it's asking you for the password. but i dont know the password.

i will reward a lot of points if someone helps me.
i think it has something to do with the old account and it was downloaded with the old account's password...