just edit the following line in config file..
password <the new password here>
Also if you want you can use encrypted password. To generate encrypted password enter the grub command line, either by pressing 'c' in the menu or by running 'grub' under Linux, then run the command 'md5crypt'. Enter your password, it will give you the encrypted form.
and now the line should like
password --md5 <now here the encrypted password>
Hope I am clear !