Is it ethical to accept masters admission from two different countries? I have obtained admission from a well reputed university X from lets say Country A, and I also accepted their offer. Now I have also applied to another university Y, from country B, but which still did not publish their admission decision. Now after a while I got an offer for admission from University Y and for some reason I want to switch from university X to university Y.

Do I need to tell university X that I am not interested in their decision? Is it even possible?
It does not matter whether the universities are in different countries, continents, cities, states, provinces or buildings. In accepting an offer of admission from one university, you have promised them something in return for that offer -- that you will go there and pursue a course of study. Since it is not possible for you to accept a second offer while still satisfying your promise to the first university, you are ethically bound to the first university, regardless of there being a second university that is also interested in you.

Consider the situation had the tables been turned -- a university seems to want you, and offers you admission and financial aid, which you accept. Then the university finds another candidate which it wants even more, and withdraws its promise. Would that be ethical? I have a hard time imagining that anyone would say that it is.

This does not mean that you have to silently accept the consequences of your decision to accept. You can very honestly inform them that you have a better competing offer which you would prefer, but you recognize that you have accepted the earlier offer which creates an obligation for you to perform. Still, you hope that the university / department would be willing to release you from the obligation, though you will make good on your promise if they will not do so.

EDIT: The below-mentioned CGS resolution is potentially relevant, in that the first university may have made an offer accompanied by such an "escape clause" (assuming it was an American university). But I assume that you have no knowledge of there being such a condition extended by the offering university, otherwise you would have mentioned it. The fundamental moral question that you need to resolve is whether you believe that your promise becomes null and void if you discover that the university had an limitation on the normal understanding on "acceptance". Unless at least one of the involved universities is a CGS institution, this resolution wouldn't really affect my ethical point.