Change of start date for rail pass

Hi,

I am just contacting you in relation to my recent booking of the Japan Railpass. I have just realised that I have made the reservation for the wrong date. I have booked a 14 day pass from July 6th 2018. My intention was to book a 14 day pass from July 20th. Would you be able to advise me on how to amend our booking with these dates? I would greatly appreciate it if this could be done at your earliest convenience.

Thank you in advance for your help.

Kind regards,

David