Just send an email from the address you used to register your account to support@haveibeenpwned.com and specify the new address you'd like included on your subscription. Please also include as much detail as possible that can be used to verify the legitimacy of the request, for example the payment method, billing date and last 4 digits of the card used.
It is strongly recommended that if you work within an organisation, you change the email address to a shared mailbox and manage the members within there internally. This ensures you won't lose access to your account or have to manually log tickets when colleagues change roles or move on.