You'll need to use an Edit step. One good place to start is with something available in any edit step. Saturation/Warmth for example. At the bottom of the step click the triangle next to the word Opacity. Then click the RGB dropdown box. There you can adjust opacity for each color channel. You could reduce the saturation and turn the opacity for the green and blue channels to 0 (zero), thus reducing saturation only in the red channel.
You can also use a Levels & Curves edit step, which has an RGB dropdown immediately available at the top of the step.
The LCH editor would be worth looking into too, it's quite versatile.