As soon as the reschedulement of the event has been announced by the event promoter or by Ticketmaster, you can request a voucher via the link in the email. You will find instructions about this in the same email. It will take up to 4 weeks before you can see that your voucher request is being processed in your account. In case of a cancellation you will automatically receive a voucher, unless you request a refund.
You will in any case be informed proactively by email at the moment you can request a refund or voucher. From that moment you have 4 weeks to fulfil your request.
It may be that your event has been rescheduled before and that this has already been announced by email. You will then receive another email with more information and instructions about this process.