Simply replace the applications in the main folder
with the new applications contained in the dmg file.
Make sure you have a copy of the folder containing the applications
on your computer.
Download the appropriate update on to your computer.
Open the folder and delete the following files; metronome.osx,
You will also need to delete the main program which will be named
firstSteps.osx, GuitarCourse.osx, GuitarCourse2.osx or something
Copy the files from the update in to the folder and arrange then
as you find best.
You should now be able to run the software as normal.