From the FAQ section of WY website:
How can I use my complimentary upgrade voucher?
Login to your account, click on the 'Upgrade reward' tab under 'Spend your miles' option to view the details of the vouchers awarded to you. Select the voucher you wish to use, update the ticket details of the beneficiary and submit your request.If the seat is available, the Upgrade will be confirmed within 3 working days and you will be notified by Sindbad Service Centre team. Additional taxes applicable, if any, will have to be paid at the time of reissue of the ticket.Your ticket with confirmed booking in Business Class will be emailed to you on completion of above process to the email registered in your account