You need to be root user in order to be able to edit that file. Either login as root and proceed, or type su in a console and give the root password. In the file, you will see that there are many different OS's listed below. There will also be a line saying "default x", where x is the position of the OS which is to be loaded by default. Change this, and save the file.
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Hardware - Celeron 266 MHz, 128MB RAM, Intel i740 8MB RAM
Software - Slackware Linux 10.1; Archlinux 0.7; Windows 98 SE
Humanware - GS/CS C++ L+++ w b+++ DI+ D+ e
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