Chapter 1 (titled "Getting Started") section of the pilots guide/manual has some pretty good tips on operation of the box and the various operations of the box...a very informative place to start if you're going to be using said box in the air. Note that you can change the frequency by both knob and/or typing it in and you touch the COM or NAV window to flip/flop between ACTIVE and STBY freq's (or hold the small right knob).
There is a ton of information in the free manuals like the pilot guide and cockpit reference guide that you can see & download HERE. Should answer most of your questions and then some!