To silence the ringer/vibration of an incoming call, you can either press either volume button (+ or -) or the "Sleep/Wake" (Power) button once. The call will continue to ring until it is ended or forwarded to voicemail. The caller will not be aware of your action.
Note that this only silences the ring of the particular incoming call. For all calls, you can also set your phone to vibration mode by toggling the “Ring/Silent” switch located above the volume control.
Alternatively, with Bria CallKit integration, you can decline the incoming call, by pressing the “Sleep/Wake” (Power) button twice (pressing once will silence the call and pressing again will decline the call).