Well ... let's see: First of all, you only can change
green loops. When you double-click them, the editor window opens in the lower part of the gb screen. You can see the beats as bars in a grid. From left to right you see the time line, separated into bars, quarter notes, eight notes etc., depending on the magnification that you set with the ruler on the far left side.
Quarter notes are the ones that you normally count "one, two, three, four", eighth notes are half of that etc. So these little bars tell you where a note begins and where it ends (in case of drum sounds, the length doesn't matter, each note triggers one "beat" on the drum).
On the left side of the window, you see a piano keyboard sideways. So the horizontal lines refer to the respective keys of the keyboard, normally the pitch of the sound. With drums, it's different: Each piano key (and every horizontal line) represents a drum sound, like bass drum, snare, closed hi-hat, open hi-hat, cymbals ... you can hear the sound by clicking on the piano key. Almost every drumset also has a rimshot snare sound, just try to find it! (In GB's Rock Set, it's on C#1, the black key just above C1.) Listen to the loop and watch the cursor go by: You literally see the bass drum, the snare etc. do their stuff.
So if you only want the snare, e.g., just go ahead and delete everything that's not on the snare's line. You can move all the notes around in time and create new ones: press the apple key, and your arrow cursor turns into a pencil.
That should give you something to start playing around a bit. "Quantizing" will be lesson 2.