Freckle is adaptive, meaning that as students practice, Freckle will automatically adjust their level up and down. This generally means that letting students practice will ensure that they are at their appropriate level.
However, if you want to reset your students progress, you can do so through the Manage Rosters option in the top left of your teacher dashboard.
Click on "Student actions" next to the student's name, and choose "Reset student options".
Then, you will see the option to reset students by subject or domain.
Note that if you reset a student, ALL of their past data will be lost. Make sure you are certain that you want to reset students before doing so.