If a step has been completed but you want to do a rework on it, you can click on it and initiate an iteration.
Iterations are quite useful for when you receive new information on the goal or some feedback on your work so far. You feel like a winner by completing the step but reopen it once something needs to be improved. All activity on the iterations is shown in the activity tab so the team can follow the progress.
When you initiate a rework on a step, you can write a message to everyone in the goal to let them know about it.