Which BIOS do you have? You use a different tool for each. EvTool 1.09 for Evox M8+, XBTool 1.0.23 for X2.4981. You take a copy of your BIOS, edit it with the appropriate tool, save it, and reflash it to your Xbox.
It's easier if you use X2.5035 or iND-BIOS.5003, because you can change the fan speed by editing a text config file - no need to reflash with every change. As you'll need to reflash to make the changes I'd suggest you change to one of those BIOSes, then you can tweak the config easily. Which you choose depends on the size of your chip or TSOP - if you only have 256k you have to use iND-BIOS.5003, if you have 512k or 1M you can use either.