I want to upgrade from xcode 6.4 to xcode 7.0

i wouldn't "upgrade", i'd keep your working Xcode version and code, separately in a repo so when and if the next version of Xcode or swift bombs and your code won't compile you can go back to a working app without headaches until apple fixes it.

i install my xcode versions side by side, just relabel Xcode.app to Xcode.6.4.app, etc. i have Xcode.6.2.app, Xcode.7.0.app and Xcode.7.1.beta.1.app all work without issues. if you really want to have Xcode.app, just make a soft link to is with ln -s

The general process should be as simple as loging into the Mac App Store and installing the latest version of Xcode from there.