You could reschedule the AZ-900 exam by logging in to your Pearson VUE account, and there you will see on your home page, the upcoming appointments, there is you have already registered for the AZ-900 exam, you will find it there. Just click on the AZ-900 that you would like to reschedule or cancel. After you click for rescheduling, you will then be prompted for fixing a new date, fix it and confirm your rescheduling.
If you are not prepared for the AZ-900 exam, I better suggest you take up the AZ-900 certification course, which offers you instructor-led training and guided projects to help you become a qualified professional to better face the exam. Have a look at our YouTube video on AZ 900 Certification Exam to help you get started.