I am using company's computer. When I tried to upgrade xcode, it pops up someone else's account. Can I change it to my email address (apple id), so I can upgrade xcode. I heard that I need to reinstall the computer can do that (but I don't want to reinstall it)
Ask your employer to enter their administrators password so you can upgrade Xcode.
It sounds like your employer set you up with a normal account that needs an administers permission to do certain things, like update apps.