How to downgrade Xcode to previous version?

When you log in to your developer account, you can find a link at the bottom of the download section for Xcode that says "Looking for an older version of Xcode?". In there you can find download links to older versions of Xcode and other developer tools

I'm assuming you are having at least OSX 10.7, so go ahead into the applications folder (Click on Finder icon > On the Sidebar, you'll find "Applications", click on it ), delete the "Xcode" icon. That will remove Xcode from your system completely. Restart your mac.

Now go to Xcode Releases page or Apple Developer More Downloads and download an older version of Xcode, as needed and install. You need an Apple ID to login to that portal.

After install, you can also switch between multiple versions:

Xcode > Preferences > Locations > Command Line Tools