How can I change the exposures, or f-stops on the Nikon N75 35mm camera?

your on camera settings in Exposure has the multiple exposure option On.

It sounds like you've got exposure bracketing turned on.
Refer to the "Bracketing" subsection of the "Exposure" section of the manual (page 130 in my copy). If you need a manual, you may download a copy here.

The Rokinon lens lacks the electronics to communicate with the camera. You need to set the camera to the Manual exposure mode. Since the mirror lens also lacks an aperture control, you will need to control the exposure by changing the shutter speed and/or the ISO setting. Also, depending on the model of your Nikon, you may or may not get any exposure assistance from the camera's light meter. If you don't, you can use the image review on the LCD and the histogram to tune your exposure.

The lens lacks the electronics to communicate with the camera. You must switch the camera to Manual exposure mode by turning the mode dial to M. The lens has only aperture, f/8, and so you have to control the exposure by changing the shutter speed and/or ISO. Unfortunately, the D90's light meter will not give you any help here.

You can get a general idea of the exposure by using another lens before mounting the 500mm. From there you can fine-tune the exposure by using the histogram display on the LCD monitor. This is how I do it with my D90 and 500mm lens.

Depends on the zoom length. If you check exposure at zoom 18 mm then zoom to (lets say) 100 for the final picture the flash will zoom concentrating the flash power as well; the flash gets stronger. This is all explained in the SB-800 manual. On a zoom such as the 18-200 the more you zoom the smaller the f/stop. You are better off with a zoom that maintains its f/stop through the entire zoom range. That way when you check exposure at 18 mm lets say f/2.8 then zoom in it will maintain the same f/stop. Myself I am not a big fan of zooms especially ones when zoomed change to a smaller f/stop.

The EM doesn't really have a manual shutter speed setting. It does have a Bulb setting for long exposures and a 1/90 second manual for flash, but otherwise the camera automatically sets the shutter speed to go with the currently selected aperture.

Normally you would set the aperture and let the camera set the shutter speed. You can adjust the shutter speed by pressing the exposure compensation button for +2 stops. You can also adjust the exposure by changing the ASA/ISO setting.

If you need a manual, you can download one from http://butkus.org/chinon/nikon/nikon_em/nikon_em.htm

A plus-or-minus one-third stop is perfectly normal if you're handholding the camera, especially when metering a high-contrast subject such as a lit lamp. If you really need to lock the exposure, use the exposure-lock button or set the camera to manual.

See second post

The ISO is automatically set by the DX code on your film canister - if there is no code, the camera sets ISO 100. If you load canisters yourself with, say ISO 400 film, you can adjust the exposure using the exposure compensation button at the top right side of the LCD ( " +/- " ). Using ISO 400 film set the +/- to minus 2 ( -2 ) so that it will UNDEREXPOSE 2 stops since the film is 2 stops ( 4X ) more sensitive than ISO 100 film.

Nothing to fix - this is a major switch problem Contact Nikon technical support team for advice but I fear it is new camera time