Here's the script:
on runI saved it as an Application from Script Editor. I then grabbed an image of the "Back" button in the Finder toolbar and rotated it 90 degrees clockwise so the arrow points up. Paste the rotated image as the icon for the "Up" script, then drag the "Up" script to the Finder toolbar. I now have a working "Up" button in the toolbar.
tell application "Finder"
try
activate
set this_folder to (the target of the front window) as alias
set the target of the front window to (container of this_folder)
on error error_message
beep
display dialog error_message buttons {"Cancel"} default button 1
end try
end tell
end run

