voltage range of ac signal
My understanding is that to add a positive Offset, you add a Postive DC voltage to an AC signal which gives the signal a positive offset. So to shift it down, what you will have to do is give it a negative DC voltage. You can look up how to provide negative DC to power an opamp, and use the same way to generate a voltage of -0.5V and combine it with the signal. 
The diode idea by Dwayne should also do the trick. 

Another thing you can do is bring it down to 0, by adding a capacitor or any other way, so it fluctuates by an equal amount like normal AC around 0, and after that you can add the positive DC offset to that signal. But this will require more circuitry, but should do the trick.