Even though Construct 2 has great support for various mobile platforms, exporting to these platforms won't be easy. You still need to download additional software called development kit from each platform holder and use it to build your mobile game. The easiest way to publish for mobile is, perhaps, to export as an Open Web App and submit the game to the Firefox marketplace, only supporting mobile devices.
Note
If you want to read a lengthy tutorial on it, there's one on Scirra's website at https://www.scirra.com/tutorials/74/publishing-and-promoting-your-construct-2-game.
Another way to export a game for mobile platforms is to use the PhoneGap export option. PhoneGap is a framework that builds your HTML5 game to be ready for the three major platforms: Android, iOS, and Windows Phone. It also has a service called PhoneGap build service that builds our game in their cloud server; we only need to supply the HTML5 files.