There are [6] answers for your question [Book].

Number subject
1   How to book tickets?
2   Is it available for nonmembers to book tickets?
3   Can I use my existing YES24 ID at english-based booking site?
4   Until when can I book tickets and How many tickets can I purchase at a time?
5   Can I change the dates and hours for advance purchase tickets?
6   Ticket booking charges booking fee?
If you would like more detailed information in addition to FAQ Customer Center-mail (global_yesticket@yes24.com) to
Your name, contact information, e-mail address, along with Purchase Number, please send contact information.
Within 2 to 3 working days can answer that.