When you find that you've assigned the wrong assignment, the very first thing you should do is click ASSIGN on the correct one. This will deal with the immediate problem and give you some time to adjust. Though your students are now working on the correct assignment, as soon as they get to Mixed Review, they will see the problems you did not mean to include.
To remove the skill(s) from your students' Mixed Review
Choose CLASSES from the blue ribbon.
Choose the class from the left column.
Choose Mixed Review from the middle column.
Find the skill(s) in the list.
Move the slider to the left to deactivate the skill.