What exam board are you doing?
Higher maths is challenging and there is a lot of content in it you probably have not covered before. Law and dentistry are quite different carreers - what made you switch to dentistry? I'm just curious because I'm very interested in dentistry too and hope to eventually study orthodontics!
In terms of your grades: are you a very high 5 or a low 5? If you're a very high 5 (and I mean near 100% in all of your maths mocks and assessments) then I think you could be able to take higher, although this is very risky and I can tell you now you definitely won't be able to cover all the topics, this is coming from a higher Maths student. If you pick a few topics to master, again this is also risky because there's no guarantee they're going to come up!
I think that in your position, I would be very stressed trying to learn everything in a month along with all my other subjects even though I scored all 8s and 9s in my mocks. Do you find it easy to motivate yourself or do you get distracted easily when working? These are all things you need to take into account.
Overall, I wouldn't take higher even if you are a high grade 5 because of the stress and the fact that you will be at a massive disadvantage compared to 80% of the other applicants this year. Also, remember there's loads of jobs within dentistry - not just a dentist! You could be a dental nurse or technician which I think may have slightly lower entrance requirements (though double check this!) Also, your dream job may change! Generally law graduates are people who study essay based subjects (I do recognise this is not always the case!) But you definitely need sciences for dentistry. What subjects are you thinking of taking next year?