garmin 800, customise main screen

Location
Norfolk
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
DISTANCE next
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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