i have to change my board from rbse to cbse but in the same city so what are the formalities

Hello.
Yes, you can change your board from RBSE to CBSE but it is somewhere depend upon the class in which you want to change your board, in class 11 after your class 10th then It would be easy to change ,some schools take your admission on the basis of their own entrance exam while some schools do it directly through your 10th board score , her during admission you have to submit your Transfer certificate and bonafide certificate and migration certificate to your school in which you are going to take admission which may be either of the same board or may be of the different board, now if you want to change your board directly in 10th or 12th (Board exams held) so it would create problems , it may be quite difficult because CBSE has fully prohibited to do not take direct admission in class 10 and 12th , it is strict guidelines for all the schools affiliated to CBSE , even for changing your board in these classes along with the above procedure you will have to give proper reason to the board through an application that why you are changing your board and it will be processed if your reason looks genuine and get verified by them ,yes in these classes if you don't have migration certificate then you will have to apply for it else in other classes (in which board exams do not held )  it will be easy to change board through documents mentioned above directly.
Best of luck.