You can edit it by this method

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Step 1

Boot Ubuntu, login as a user and start a terminal session.
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Step 2

Make a copy of the GRUB menu file as a backup by entering "cp
/boot/grub/menu.lst /boot/grub/menu.lst_BU." Use this file to restore
menu.lst if the changes fail.
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Step 3

Edit the menu using the command "sudo gedit /boot/grub/menu.lst."
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Step 4

Count the number of possible operating system choices. For example, when
first installed Ubuntu will be the first O/S in the file. Lower down in
the file will be Windows XP. Count other operating systems, if installed.
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Step 5

Find the line "default = 0." Grub sets the default to boot the O/S
located as the first (0 relative) in the file menu.lst.
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Step 6

Change the line "default = 0" to "default = 1." Save the file and exit.
This changes the default from Ubuntu to Windows XP. Ubuntu appears first
in the file and first on the menu display. Windows XP is further down in
the file and appears below Ubuntu on the menu display.
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Step 7

Restart the PC. Let the GRUB menu take the default to verify the change
. The default is now set to 1 which means Windows will boot when no
choice is made.