One thing to ask yourself is "which fan is making all the noise?" A typical desktop computer has one fan in the front, a fan (or several) in the power supply, one on the CPU, and probably one or two on the video card. Some of them can be controlled from the motherboard BIOS settings, but if you don't know what you're doing, it's probably not a good idea to mess with them. I really wouldn't recommend altering the fan to run less often. You will probably shorten the system's lifespan this way.
Some of these fans can be replaced with "quiet" fans, which isn't hard to do. This is probably the best solution, but bear in mind that they are not as effective because they spin more slowly. The more powerful fans tend to make more noise. Does it sound like it is damaged or wearing out? Has it always sounded this way? Are you sure it's a fan and not some other part, like the hard drive?
If it's a laptop, you probably can't do much for it at all. Otherwise, open the case with the system running (don't touch anything!) and listen for the source of the noise. Then look into replacing that fan with a quieter, lower RPM model.
IsaacJ