Eu estava procurando a mesma coisa recentemente.
Actually very simple. Use a text editor as "root" and edit the file /boot/grub/grub.conf to change defaults (no graphic config program for grub).
Look for the lines: default=0
timeout=10
or something similar.Change the "default" number to whichever position your Windows boot info is in your list (start counting at 0), and reboot.
Since you only have the two OSs, probably just change to default=1 and you shoudl be all set. If you want to shorten the delay before the default kicks in, "timeout" is the number of seconds delay.
Good luck.
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