Allowed but not recommended. Put bracket either side and put a line through. If you think you made a mistake and tipexx it but it was right you can't get any marks. If you put a line through it you might.
No tippex in exams. Ever. Period. Just cross out your answer and write a new one. That way, if your new answer is wrong but your old one is right, they can try and give you some marks. If it's tippexed out, you can't get any marks. I just cross out my answer and put it in brackets. The wisest choice, trust me.
the new way of marking, by scanning the exams into computers to be digitally corrected asks that you dont use tippex. however CSPE is marked via the old scheme so its allowed for the exam. hope this helps :-)