I made a mistake in one of my Challenges that may be linked to something i did during an execrise in the lesson.  I see tips on the forum to use a fresh Trailhead Playground, but it's not clear how I do that.  I keep winding up in the previous one and getting error messages due to the mistake I may have mad previously.

Hello.  I am in the Admin Beginner Module in Trailhead.  I had just started the Data Modeling module, did the exercises in the first lesson Understand Common and Standard Objects.  But in the Challenge, I must have done something wrong, because I got an error message.  The problem is I can't figure out how to wipe the slate clean and start over again.  And when i try to do the next lesson Create Object Relationsjps, I can't advance in this because of the prior error.    

Hello.  I am in the Admin Beginner Module in Trailhead.  I had just started the Data Modeling module, did the exercises in the first lesson Understand Common and Standard Objects.  But in the Challenge, I must have done something wrong, because I got an error message.  The problem is I can't figure out how to wipe the slate clean and start over again.  And when i try to do the next lesson Create Object Relationsjps, I can't advance in this because of the prior error.