How can I access all the practice sets for each stage?

I'm only on Stage 1 -- once I had passed eight of the practice sets, it's now making me continue onto Stage 2.  I was wanting to go through every single practice set in Stage 1 before continuing onto the next Stage.  How can I go back to Stage 1 and complete the sets I never started? It's only showing me the eight problem sets that I did do.