Crack the front panel open slightly. 1cm is enough.

At the top of the Omni just behind the screen is a red lever, press this in.

While holding the lever down, press the 'Prog' button on the front panel.

You should get a boot menu appear on the screen

Release the lever and the 'Prog' button 

Choose 'Select BootImage'

Select the other BootImage and say 'Y'

The Omni will reset the RAM and load the boot image, you shouldn't need to do anything else