Currently, practice assignments cannot be reset since they are based on each student's mastery level. Mastery automatically resets each summer, but cannot be manually reset during the school year. While teachers cannot manually reset practice assignments or mastery levels at this time, they can always create new practice assignments when students need additional work on a topic.
Students who have already mastered a topic will receive a shorter Refresher Quiz at the start of the assignment. During these refresher quizzes, an incorrect answer may cause them to drop one mastery level, requiring them to complete a full set of questions to regain their previous level. You can find more information about Refresher Quizzes and how they work here: What happens when students are assigned a Practice on topics they've already mastered?
Students can also practice independently at any time from the Learn page. They can access any topic for independent practice, which contributes to their overall mastery of that topic. Additionally, for topics they've already mastered this year, students can use the Quiz Me feature, found under the ellipsis menu next to each topic. For more information, check out this article: Is it possible for students to work on something I didn't assign?