Rescheduling a booking can happen in one of two ways: 1) by using the confirmation email you were sent. 2) By accessing your bookings tab if you're a Cal.com account holder. To reschedule the meeting from within your bookings tab, simply click on the meeting in question.
This will cause a new page to load with all of your meeting details. Scroll to the bottom of that page, and you should see two links, one that says cancel and one that says reschedule. Click on the link that says reschedule and follow the on-screen prompts to reschedule the meeting. If you don't have a Cal.com account, you can reschedule your meeting by accessing your appointment confirmation email.
Once you've found that email, scroll to the bottom of it, and you should see two links, one will say cancel and the other will say reschedule. Clicking on the reschedule link will open up a web page that will ask you for additional details and then allow you to submit your request for the meeting to be rescheduled.