Hello Caroline,

The support team has been completing a sweep in our Support Community and I thought it worthwhile to provide you with a reply since other people might have the same question.

Once you complete a given task at a certain level, you are not able to change the level afterward upon reviewing it. Although you should be able to change the level in the middle of the practice, and therefore start anew once you set a new difficulty level, once the practice is done, the level can't be changed.

I hope this helps!