Below is direct link to Certifications tab on your profile:
You should see your upcoming exam appointment so that you can click Reschedule, similar to below:
Your exam appointment needs to be at least 24 hours in the future to be able to reschedule it.
If above does not allow you to reschedule, please ask your question in the Microsoft Certification forum using link below. There are support moderators over there that will respond to your question in 1 business day and can assist with scheduling/profile/exam issues:
Please click Accept Answer and upvote if above was helpful.