There are at least three ways to do this, although each has the potential to go awry:
Method 1: From OS X
From OS X, you can use
bless to set rEFInd as the default boot manager. This process is described in the rEFInd documentation:
Note that's the complete manual rEFInd installation process. You really only need steps 1, 2, and 8.
The problem with this approach is that, if your rEFIt installation includes rEFItBlesser (as it does by default), it may reset rEFIt as the default boot loader. You can work around this by removing
/Library/StartupItems/rEFItBlesser. Alternatively, you can re-install rEFInd in OS X via the
refind-install script, which should give you the option of removing rEFItBlesser.
Method 2: From Ubuntu
In Ubuntu, typing
sudo efibootmgr -v will produce a list of boot options. You can then use the
-o option to
efibootmgr to change the
BootOrder line to set rEFInd as the first option. (For instance, you might type
sudo efibootmgr -o 0081,0080 to make
Boot0081 the default option, with
Boot0080 as a secondary boot option. The details of what values you type will vary from one system to another; that's why you start with
sudo efibootmgr -v -- that shows you what options exist on your computer.) This will often work on Macs, but Macs are weird, and on some models it might fail. You may also run into the rEFItBlesser problem noted in the previous approach, so you may need to remove rEFItBlesser manually. (Note that re-installing rEFInd in Ubuntu won't remove rEFItBlesser in OS X.)
Method 3: Delete rEFIt
You can completely delete rEFIt from the computer. It normally installs itself to
EFI/refit in OS X, but depending on how you installed it, it may be somewhere else, such as on the ESP. (The ESP is normally not mounted in OS X, but may be mounted at
/boot/efi in Ubuntu.) With rEFIt gone, the computer should boot to its second boot option, which may be rEFInd or OS X. If it starts booting straight to OS X, you'll need to re-install, or at least re-bless, rEFInd. There's a small chance that the computer will refuse to boot at all after you delete rEFIt. If so, holding down the Option key as you start up should produce the Mac's own built-in boot manager, which should enable you to get to OS X and recover.