What is the proper procedure for correcting a return, before the April 18 due date, that has been e-filed and accepted.

My situation is this; Filed return using standard deduction. it is later determined that taxpayer can itemize. Would like to correct/amend the return to claim a refund and hope to file before the April 18 filing deadline. I don't think i can file an amended return since it is before the filing date. Can a corrected return be paper filed with an explanation now or do i have to wait and file an amended return after April 18.

Thank you.