Hello.
If you are a science student and you have done your class 12th in the same stream and now you want to pursue bachelor course in art in your college that is is very easy you don't need to do anything you will have to just apply for the course in your selected University/College, and simply after getting selection you can do it but still in the starting level you will face some difficulties because you don't have knowledge of the subject which you should had in your class 11th and 12th so you will start directly from advance topic in your bachelor so for this you can brush up yourself and do hard work by doing some extra reading.
Best of luck.