garmin 800, customise main screen

does anybody know how to customise the main screen? at the minute i'm quite happy with my display apart from one thing, at the minute i have
TIME at the top
SPEED on the next row
and on the last row in the left hand side i have TIME OF DAY, but on the right hand side i have CALORIES, instead of this i would like AVERAGE SPEED, could any of you kind folk point me in the right direction?
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