I do not think you can manually change fan speed on this Dell model.  Its BIOS does not have such option.  you may disable some advanced cpu performance feature in bios and attempt to decrease cpu fan speed indirectly.  that may be some leverage over cpu fan. Default: Enabled. Default: Enabled. Default: Enabled. Default: Enabled. Default: Enabled. Inspiron 3881 Service Manual | Dell US
How do i change my cpu fan settings on my dell inspiron 3881
Cpu fan