The main tab window shows the current track tab. And allows you to edit it.
Use your mouse and arrow keys to move the "cursor" (the yellow box).
Use your numbers to edit the notes.
Std Windows keyboard shortcuts work the same.
Crtl + Z - Undo
Ctrl + X - Cut
Ctrl + C - Copy
Ctrl + V - Paste
etc
Left click and drag to select notes/bars, the selected bars will be highlighted blue. As shown below
Theres a numbe of keyboard shortcuts in GP, the simplest way to find these is to hold your mouse over the toolbar button you want, and wait for the "tooltip" to show up, this will generally show the associated shortcut.
But a few to get you started:
A - Chord Window
B - Bend Window
H - Hammer on/Pull off
V - Vibrato
+ and - Increase/Decrease note duration
P - Palm Mute
I - Let ring
S -
Slide
Ctrl + U - Upstrum
Ctrl + D - Downstrum