Hi.
The fan in your computer works automatically to cool down the internal components to prevent them from overheating. To slow down the fan, you need to cool down your computer. You can do this by putting less strain on your computer, by doing things like deleting unwanted programs, removing viruses, and limiting the amounts of processes running on your computer.
Here’s a guide:
https://www.google.com/amp/s/www.pcmag.com/article2/0,2817,2364937,00.asp%3famp=1