It's pretty easy, but don't mess up or it won't boot.
Look at /boot/grub/menu.lst (grub 1) or /boot/grub/grub.cfg (for grub2, which you probably have). I'll call these the "grub file".
NOTE: You're "supposed" to edit some other files, then run a grub command to update the files above.
I find it a lot easier to just edit the files...
Anyway, make a backup copy of the file you have. (menu.lst or grub.cfg).
Both grubs have a "default=N", where the nth entry in the "grub file" is what gets booted by default (starts with "0" for grub1, not sure about grub2 - you can prolly figger it out!)
As for multiple entries, you can just delete the extra ones; each one will be in one chunk of contiguous lines in the file.