Can't turn a line into a construction after finishing sketch

[Anonymous]

I made a sketch of a circle, and then made a revolve using it to get a sphere (I know I should have used a sphere form, I didn't know it existed at the time). When I finished the larger form I was working on, I noticed the line used in my initial sketch was still visible. From my understanding, making it into a construction should fix this problem, but I can't access the sketch pallete because I already finished the sketch. 

[jeff_strater]

Select the sketch, right click, and choose "Edit Sketch".  You cannot change a curve to construction unless you are actively editing that sketch.