I accepted a PhD offer in feb. of this year. now i have been given a far better offer by another institution. what can i legally do?

Well I think it really depends on the kind of offer you accepted, and any terms and conditions it had. Absent any of this info in your post, I would say that it shouldn't be a problem to write to the university (or supervisor) concerned and explain that you had been intending to take up your place, but another institution has made you a funded (I assume) offer, and you have decided to take that offer instead. My first PhD offer came in December and they gave me two weeks to accept or decline (and I couldn't apply for funding with them until I had accepted), but my other offers weren't due to come in for a few more months. I accepted the offer, but have since received full funding at another institution and so wrote to the supervisor I had been assigned and explained the situation honestly. He was totally fine with it. But, it depends on the terms and conditions of the offer you accepted.