1. Click and drag on the four-arrow icon in the upper-right corner of the lesson box, and release on the lesson you wish to merge to. 

2. Select "Link" when prompted to link the two lessons together, displaying the contents of both lessons. 

If you wish to "Unlink" the lessons at any time in the future while still retaining the merged information, right-click the lesson, select "Unlink Lesson", and select "Yes".