When you click on a completed step you are asked if you want to make an iteration to this step. This means that you can go back in the process and do a rework on this step. Maybe a design didn't get improved and now based on the feedback, you can rework it. Maybe the report you were writing needs a few more details to be added.
Every time, you have to rework a something, you can start an iteration on a step and so through it again. Great work doesn't happen over once. With feedback from the team and the newly obtained knowledge you can keep a sense progressing but still make changes that will improve the final results.