When a payment is successfully processed you will be routed back to the Site24x7 billing details page.
An email notification will also be sent to your registered email address with the subject Site24x7 Package Upgrade Notification. The mail will have details about your upgraded pack, the cost and other relevant details.
You can also confirm whether the upgrade has happened by verifying under Admin > Subscriptions > Billing Details. The Account Type should reflect your new pack name. Read more.