There are two simple ways to do this either via the shortcut tray or the tweaks menu.
Depending on your prefs you may see it there already.
If you dont, swipe down again for expose the rest of your shortcuts.
Mobile Data option on the tray
Somewhere on the list, youll find one labelled Mobile Data.
Tap it so you can turn your data off.
Be carefully not to accidentally turn something on/off by getting them mixed up!
Via the controls
The second, slower option, is to use the controls app.
Open it and tap the Connections button at the very top.
At the bottom of the screen, youll find the option Data Usage.
In the Data Usage menu, youll see a slider option for Mobile Data.
