You need to change fan resistor. Look under glove compartment, you will see heater fan. There are two plugs, smaller one is for the fan motor, and larger is for fan resistor. It is very easy to remove, just unplug the plug and unscrew 10mm screw, and you can take it out.
You can see location of parts in this video
But don't worry, you don't need to disassemble everything. The resistor is accessible from below the glove compartment.