Different Apple IDs

Hi,

I hope I can describe my problem in an understandable way. Here we go: I wanted to update my apps, clicked on the App store icon, clicked update, and then needed to enter my I-Tunes password. However, in that window I saw an old email address that I must have used as Apple ID in the past, I have forgotten the password to this ID. I then I went to the I-Tunes icon in the hope of changing my old ID there. I clicked on Apple ID, and saw my current ID there, not the one I saw when I wanted to update my apps...My question is, how can I change that old ID when even I-Tunes has my new ID?

I'm grateful for any hints. Thanks.