How to launch already installed app from iPhone using appium

Hi All,
I am writing an automation script using Java + TestNG + Appium.
In a test scenario, I am trying to launch already installed application on iphone. (Box in this case). Box contains some MS office file which I need to access.
I am not very sure how can I achieve this.I tried multiple options such as extracting .app file from iTunes and deploying using Appium, but no success.
Can someone please guide me, if this is possible. If yes, how?