read:
man lilo.conf
if you're using lilo. For grub, you'll need to read the corresponidng man page, I'm no sure what it is.
You need to edit a config file to reflect the new default. In lilo, this is done by changing the "Default=" line to the OS label you want booted first.
Don't forget to run your bootloader after you make changes, or they won't be applied. For lilo, just type:
lilo
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