Erasing anything on the sheet depend on the rules set by the Exam body. The instruction about filling the sheets are provided, read them carefully. Nowadays candidate can use pen on OMR test answer sheets and therefore erasing anything marked with a pen will spoil the sheet. So be careful while marking your answers.
If you are using pen to fill the answer blocks, then it is not possible to erase anything once you mark your answer with pen. So even if you mark the wrong answer, let it be. This is the only choice you have. Some exams allow pencil, you can change the answer in this case. But be careful while erasing the pencil mark. Erase the marks completely. Partially rubbed marks can get detected in the highly sensitive OMR software.
Use a soft new eraser to rub on OMR sheet.
DO NOT spread the rubbing smudge to nearby bubbles.
Properly shed of Rubbing dust before submitting the sheet.
DO NOT try to erase a Pen mark.
DO NOT cut or create holes in place of erasing. Even if you do so, it will be detected as a mark. So no point damaging the sheet.
DO NOT use whitener or blade to hide the marks on OMR Sheet. Read ore about the use of whitener.