For github web browsing, it is not shown as side by side as lots of diff tool does. Can I change the diff mode to side by side? I mean on the github web.
Nope, not possible on GitHub at the moment. You can check out other on-line diff tools, such as: http://www.quickdiff.com/, http://www.diffchecker.com/ and http://www.diffnow.com/. These will give you side-by-side diffs, but you have to copy/paste text manually.