Yes, you should report it, although given the threats against you and other related concerns of retribution or ostracism, it may be understandable if you choose not to. Certainly reporting it would benefit you personally by allowing your work to be evaluated fairly compared to that of others, similarly benefit other honest and hardworking students in your class, benefit your institution's academic reputation, and benefit your own conscience. I feel there is a strong case for reporting.
As for how, there are various means, each with their own pros and cons depending on how involved you want to get in this story, how much certainty you want to have that you can remain anonymous, and how much time it would take for your complaint to be acted upon.
For the most rapid response (which seems important given that the exam is in two days' time), I would draft a polite email to the instructor, along the lines of:
Dear Professor ExamRecycler,
I am writing to express my concerns that some students in our
class have been memorizing solutions to your class exam from last
year after you indicated that you plan to give the same exam to our
class. I saw some students doing this and have suspicions that the
phenomenon could be a lot more widespread. As an honest student who
studies hard, I think I have a right to expect that my performance be
graded based on a comparison with the true performance of other
students, rather than a fake performance attained through dishonest
means. I would greatly appreciate it if on Wednesday you can give us an
original exam that does an honest job of testing our knowledge.
I also know that there are many other honest students like me in the class and
I'm sure they would all share my sentiments about this.
By the way, thanks for a great semester! I really enjoyed your course and
learned a lot.
You can make some small variations to the email depending on how much you trust the professor to follow up on your request (and how receptive she would be to the shameless flattery at the end...). For example, if you feel she is not likely to bother preparing an original exam despite your request, cc'ing the email to her department chair could greatly increase the chances that she would comply. Alternatively, you can indicate as diplomatically as possible that you are considering complaining to your university's Student Judicial Affairs office, etc.
Another possibility, if you really don't trust the professor to take kindly to the request, is to go directly to the department chair (or undergraduate program chair or advisor), or to Student Judicial Affairs. Note that this would likely mean a delay in getting the problem addressed, so in particular it's unlikely that anything would get done before your exam on Wednesday.
There is also the question of whether you want to expose the names of the students who you believe are cheating. I don't have good advice to offer about that. Note that they can reasonably claim that looking over old exams is not cheating but a form of studying. It is really your professor who is behaving somewhat unethically by reusing the same exam and openly declaring that fact. In my institution, the Code of Academic Conduct explicitly says that faculty have a responsibility to "Use examination formats that discourage academic misconduct". As a member of my institution's Campus Judicial Board, I attended hearings for students accused of misconduct, and on more than one occasion felt that the faculty had been negligent of that responsibility and therefore bore part of the blame for what happened. I have an impression that this may be the case in your situation as well.
Finally, although you didn't ask about this, I must say I am very strongly bothered and concerned by the fact that you received threats (of what sounds like physical violence or other serious harm) on Yik Yak. This behavior is much more disturbing and egregious than the cheating, and if I were you I would immediately report it to Student Judicial Affairs or another unit on campus that could provide you counseling about keeping yourself safe, protection if needed, and who would be strongly motivated to seek out the people who threatened you and take disciplinary action against them. Such behavior has no place on a university campus and must absolutely not be tolerated.