Which ROM? If you have the zip of the ROM, unzip it, navigate to /system/media and find the bootanimation.zip. Then on your phone navigate to /system/data with an app like root explorer and replace the old with the new. Make sure to set the correct permissions, it should be the same as everything else in that folder.
Tapatalkin it from my IHO CM7.1'd, ICS Themed Optimus V