To view your past lesson plan content from a previous semester, there are a few things you can do!
Recommended Approach: Unit View
Navigate to the Unit view, and then click on the left arrow beside the date.
That will change the semester to be the previous one, and you can now select a class from the dropdown.
Now, you will see a list of all the lessons for that class, for that previous semester. That's it!
Alternate Approach: Navigate to the past date
If you instead wish to see your previous semester's lessons from the context of the Day or Week view, all you need to do is simply navigate to the past dates of that semester! For example, if my Year 1 semester was in 2016 and it is currently 2017, navigate to a day/week in 2016 in Planboard and voila, it'll show your old lessons!