Wow, I looked all through the User's Manual and couldn't find the answer. Finally I had to contact the manufacturer, and I was able to obtain the steps:
From the Apps Menu (not the homescreen - for some reason this won't work from the homescreen) press and hold the app (in this case your Email app)
When you return to the homescreen, drag the shortcut icon over the Quick Key icon you would like to replace, and let go.
Hope this helps!