You are using the global transposition for horn players instead of the regular (soft) transposition.

 

First you should turn off global transposition for horn players:

iOS: On the top of your screen in song view, tap on the third button from the right, Aa, (Chord Chart Settings) , then Transposing instrument and select C.
Android: in the app under More, tap Transposing Instrument and choose C.

OS X: in the app Settings (Preferences), choose C in the Transposing Instrument drop down menu.

Windows: Settings, choose C in the Transposing Instrument

 

Then you can transpose normally:
iOS: Tap on the chart, then the key in the player controls at the bottom and select a new key.

Android: Tap the Key on the left and select a new key.

OS X: Click on the key in the Player controls (right top) (or bottom right if Player controls are minimized) and choose a new key

Windows: click the key in Player controls and choose a new key

 

Detailed information

iOS, Android Soft transposition

Mac, Windows Soft transposition