If they're plugged into fan headers on you MB, there should be controls in BIOS. If your BIOS is not tweak friendly (eg, Dell), an app like speedfan might do it for you depending on the type of controller used.
If it's not plugged into your MB, I'd first have to ask why?... then if there's a good reason for that, you might want to look at a hardware fan controller.
JD
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