Well, you can easily use it in your update method by assigning it in a script property like this:
this.app.mouse.on(pc.EVENT_MOUSEUP, function(event){
if (event.button === pc.MOUSEBUTTON_LEFT) {
this.mouseLeftReleased = true;
}
}, this);
And make sure to clear it at the end of your update loop:
MyScript.prototype.update = function(dt){
// update loop end
this.mouseLeftReleased = false;
};