Looks like the information you desire is on page 147...
"Press Menu botton--->Setup menus--->Format memory/format card" Formatting the Internal Memory To format the internal memory, remove the memory card from the camera. The Format Memory option appears in the setup menu.
Hope this helps!
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http://www.camera-usermanual.com/nikon-coolpix-p90.html
Looks like the information you desire is on page 147...
"Press Menu botton--->Setup menus--->Format memory/format card"
Formatting the Internal Memory
To format the internal memory, remove the memory card from the camera. The Format Memory
option appears in the setup menu.
Hope this helps!