Turn Edit mode on and tap the Open course index icon that can be found on the left side of the course page.

Course index is a list of all your Sections and activities in your course. The index opens on the left side of your course page. When you hover the mouse cursor on any activity on the index, an arrow icon appears. Grab either the arrow or the name of the section/activity you want to move and drag it on top of the section name you want to move it.