How do you stop a plan and start a new one?
Two that are probably good starting points for you:
https://support.trainerroad.com/hc/en-us/articles/360016201211-Add-Your-Current-TrainerRoad-Plan-
Yeah, but how do you stop a plan midway and change to another plan?
This is the other half (I posted those since I found them first, but was still searching for this).
This is the overall Calendar help list, for other useful operations:
https://support.trainerroad.com/hc/en-us/sections/360002267331-Calendar
Fantastic, thanks for the help
Cheers 
One more question sorry, how do I modify the days that each workout of a plan is currently scheduled for. e.g. if I originally selected workouts every monday, wednesday and friday, how to I change it so that they are every monday, wednesday and Saturday?
If you delete the plan but have already completed x weeks, does it delete that , or just all future non completed workouts?
It’s best to set those preferred days when you add the plan. It will do all in one shot.
You can either drag the ones in your existing plan to the desired day (one at a time). Or you can delete the plan and swap the days during that step.
Start at 19:00 in this video.
Good question. I have not tried that or seen what happens.