Most students should be able to determine their proper placement from the Placement Page; and students with borderline or otherwise complicated cases can get advice from the SFI Placement Advice page. The SFI is also usually available for consultations before the start of the term. All of these resources can and should be accessed before classes begin; so, most students should start the term in the correct class.
The first week of classes is then an opportunity for students to confirm their placement. Students who might want to change to a different class can do so for the entire first week. Note, this is a significant amount of registration flexibility afforded to students, because in this first week classes are covering material, doing group work, and completing graded items both in class and out of class. Because of this significant and important activity, it is important for students to try to get into the proper class as early as possible in that first week; joining a math class late in the first week will require a significant effort, even for students with a previous exposure to similar high school or college courses.
It is the responsibility of the student to ensure by the end of the first week that he or she has the necessary background, qualifications, and resources that will be needed to do well in the math class in which he or she is enrolled.
By the beginning of the second week, courses in the Mathematics Department have moved forward to an extent that joining the course in a way that is feasible, fair to the other students, and consistent with the philosophy of the course might not be possible. Either the course instructor or the Mathematics Department might disallow such a request.
Dropping Back to a Lower Course
Some departments allow students to "drop back" to a lower level course with a later deadline than that for students who have not been attending a course in that department. Such a policy, when feasible, can be encouraging and convenient for students who are not entirely confident in their placement choice.
However, due to the tailored content and assigned work of each individual course, such an option is not realistic for lab calculus courses in the Mathematics Department. Specifically, having attended a higher level course and attempting to "drop back" to a lower course (or, similarly, "jumping up" to a higher level course) is usually not much easier (or more likely to be allowed) than simply adding that course late in the drop/add period.
The lab work in these courses is central to the course structure and philosophy, and is not part of the calculus curriculum of most high schools or other universities. Students who have missed even one lab meeting will have significant difficulty in catching up in the material and work of the course fully and fairly. Students who have missed more than one lab meeting are unlikely to be allowed into the course.
Strategies to Succeed
If you are less than confident that you have the appropriate preparation for and resources to dedicate to taking the math course you have selected, you should consider attending and participating in BOTH that course and the lower level course that you might need to consider dropping down to. Note, you can attend such a course and participate in the work even without being registered (though of course you will need to have the lab manual in order to participate!). This participation will significantly extend the point in the term by which you will be able and allowed to change your mind to take the lower level course. And, of course, if you decide that your initial higher placement is satisfactory, then your continued participation in the lower level course is not necessary.