Here's how to remove iTunes from Windows Vista or Windows 7:
Here's how to remove iTunes from Windows XP:
Removing and Reinstalling iTunes, QuickTime, and other software components for Windows XP
Here's the iTunes 64 bit download:
You do not uninstall itunes on thephone you do that on the PC. So you have a PC and I assume that it 64 Bit operating system. go to control panel and uninstall the 32 bit version You uninstall by clicking on programs and feature in WIN 7. Similar place in Vista or XP. I am sure you will find it. If the 32 bit version of itunes doesn't completely uninstall then read this https://discussions.apple.com/message/22193251#22193251
Then download and install the 64 bit version of itunes.
You cannot merge user names. Remember the things you bought with the old ID are forever linked to that ID an dthose apps can only be updated using the old ID. You could delete those things and download (and pay again) those things again using the new ID if it is a problem for you
I didn't have a problem on my PC the response to update to 64 bit was an error message on my phone when I would plug it in to the PC. I uninstalled and reinstalled on my PC all the apple stuff (this is the third time I think) and this time the installation on my PC worked without giving me a rollback error. The problem is that my computer still doesn't recognize my phone. UGGGGG and apple was supposed to be so much easier...ha ha