Click "Edit Topics"
Here you can hide topics that you don't want students to get questions on. Removing (or adding back) topics automatically adjusts the target score by 150 points.
Unchecking topics tells students that reading the e-book sections for those topics is not required, but it does not take away access to the non-required sections. Your students still have access to the entire book.
Click Save changes when you're done.
Note that you'll need to keep at least one topic. Also note that quizzes with only one topic tend to be small, and your students may experience repeating questions.