How to only select one stereo part (left or right)?

Hi, I have a stereo part and I would like to only select the left or right part of the file. Is this possible and how to do it?

You need to “split” the stereo track into separate left and right channels.
The channels can be “stuck back together again” after you have completed your changes.
See here in the manual for how to split (and rejoin) the channels: