It has been awhile since I have used MEPIS, but it has a utility in the KDE
system menu to rewrite grub. If it did not do it properly the first time
then it isn't likely to do it again, but it is worth a shot.

Otherwise, you will have to cut and paste from the menu.lst in /boot/grub of
your Ubuntu installation to your menu.lst in Mepis. You will need to use su
in Mepis to load its menu.lst found in the root directory at /boot/grub. I
think that Mepis has a file manager in administration mode in the system
menu. It used to. Use that instead or use a terminal and change to root with
su and then launch konqueror. Navigate to /boot/grub/ (in the MEPIS root
directory) and open the menu.lst in kate.

Next open the Ubuntu menu.lst. You don't have to be root so use an ordinary
file manager. Open it in kate. Copy the lines from the Ubuntu menu.lst that
you need. If you place them at the top, ahead of MEPIS, then Ubuntu will
become the default boot OS. Otherwise paste it after the MEPIS lines. Save.

Possible problem. If you have separate drives and use a boot manager to
choose which drive to boot first and both drives are set to hd(0,x) (mine
work this way, but most people's don't) then you will have to edit those
lines of menu.lst to reflect the change. Basically if they are on the the
same drive it should be no problem, but if they are on different drives then
you may have a problem when you reboot. If so you will have to manually edit
grub at boot up (press e, change and then press b to boot) and then make
changes when you get back in.