My camera is set to 400 film speed but my film is 200?

It's a Nikon FM2 - and due to its age, it's difficult to change the ISO settings [the dial is broken in someway, the last owner put sellotape across it and I don't want to remove it for fearing of something falling off]. If I use the 200 ISO film, will it turn out alright, or should I try and fix the ISO setting on the camera?

If you're using colour print film, it won't matter - the film's latitude will cope.

The setting is for the metering. It will tell you what setting should be right based on having been told your film is ISO 400. If it is really ISO 200, and you use what the camera says is right, you will be underexposed by one stop. Negative film does not tolerate underexposure as well as it does overexposure. You negative will look thin, and prints can look hazy.

What to do? Set your exposure according to the meter. Then open up one stop slower shutter speed, or one stop wider aperture.

If you're using color negative film, no problem. That's just a 1 stop difference and the film will tolerate it.