How do I switch the font back to previous in El Capitan?
You can try installing a third party application called TinkerTool.

TinkerTool is an application that gives you access to additional preference settings Apple has built into OS X. This allows to activate hidden features in the operating system and in some of the applications delivered with the system. You can download the product free of charge.

You will find the new available options in System Preferences and thus you can change the font in the Font tab as in the figure below: