Sorry, i meant the iris or aperture. If you take the holga apart there is a plate that moves in front of the regular aperture with another hole.
The second hole is usually the same or larger then the first, so it does you no good. That may not be the case with all Holgas but it is the case with the 3 that I have. As I said, the easiest way to change the aperture is to literally have 2 or 3 cameras. I have one with a broken shutter that I simply use at night or for longer exposures with a nuetral density filter held in front of the lens.