if you are going to open the case and unplug the cable i mentioned, turn off the pc, then unplug the power supply from the power cable that is connected to the wall socket, then remove the left panel of the case, unplug the rgb cable, manage that cable so doesn't fall over the cpu fan or over anything else, then close again the left panl of the case, plug back the psu power cable and turn the pc on again, if the fan spins but the led lights doesn't come on you successfully disabled the rgb, if the fan doesn't spin, you unplugged the wrong cable
to be sure what you are going to do remove the pannel and take a picture of it, post it here in the forum, so we can see the connector and guide you better on the process