You can select:
A single rung by clicking the left bus bar.
Multiple consecutive rungs by holding down the Shift button and clicking a rung, and then moving down to the final rung and clicking it.
Non-consecutive rungs by holding down the Ctrl button and clicking the desired rungs,.
After you have made your selection, click the Disable/Enable icon on the Ladder ribbon.
Disabled rungs are marked by a darker left bus bar.