The thing with the tubes is the Radiator. The fan is between it and the case. On the back of the case will be 4 screws or Pins. If you remove them the rad and fan will lower (careful and support the Rad so you dont mess up the hoses)
Between the Rad and the Fan is a adaptor.
Than remove the fan from the Adaptor/Rad and replace with a new 120mm fan.
Get a good fan like this one,