When using Mango on a computer, start by opening a course. Rather than choosing "Resume learning", you can select the Unit, Chapter, and Lesson where you'd like to begin. If you're already in a lesson, click in the upper right corner where it lists the Chapter and Lesson number to open the menu where you can choose an earlier lesson.
For the mobile apps, start by navigating to the "My Courses" screen. From there, tap the course image (not the orange resume button). Now you can select an earlier Unit, Chapter, and Lesson.