Can we change register number after complete registration and fee payment , if in case we have entered wrong register number?
If you have registered for an examination then for most examinations there are correction window options which comes up after the last date of submission of application form. Thus if the exam you are talking about have its form fill up going on then wait for a bit and as the form fill up ends its in general common that the conducting authority will open a correction window portal where you can do your necessary correction.
I hope this answer helps. All the very best for your future endeavors!