You've got several options. If you merely want to mute that section, you could Split that section out by placing the cursor at the beginning of the section and pressing S, then again at the end. You can down delete that section of the audio clip. If you want to lower it rather than removing it completely, split as above and then over the mouse at the top edge of that section. You'll see the mouse pointer turn into a pointing finger. Drag downwards and a blue volume level line will appear. The farther down you drag it, the quieter the clip will be.

For continous volume control, insert a Volume Envelope on the track. You can then add as many points as you need along this line and raise or lower them from point to point.