It's pretty quick and easy to change the layer range of a sound inside MainStage.
First, go to the menus and choose View > Show Channel Strips or click on the top right icon inside MainStage:
Then, click on the Channel Strip that matches the sound you want to change.
You should see the Instrument Channel Strip Inspector open, and then you'll want to choose the "Layer Editor" Tab. If you don't see the Instrument Channel Strip Inspector open at the bottom of the screen choose ⌘5 to show them or View > Show Inspectors.
From here, the fastest way to change the Layer Range is to play in the High Key and Low Key you'd like for the Layer Range.
First you'll choose "Learn" by the High Key and press the key you'd like on your keyboard (make sure you have your midi keyboard connected and assigned in Layout Mode before doing this) and the layer will automatically move as a result of that key press.
Then, you'll choose "Learn" by the Low Key and do the same thing.
The second way to move layers around is to hover to the far left or far right of the colored layer in the workspace, and the mouse arrow will turn into a line with an arrow which you can drag to the desired location on the keyboard.