You cant change the date of planned MCAT on AMCAS
Post-Submission Changes
Thoroughly review your application prior to submitting. After you submit your application, you can only make changes to the following sections:
ID Numbers,
Name, including Full Legal Name, Preferred Name, and Alternate Names,
Contact Information including Permanent and Preferred Mailing Addresses,
Alternate Contact Information,
Date of Birth, Birth Address, and Sex,
Letters of Evaluation (only additions of up to 10 letter entries and notifying AMCAS of a letter no longer being sent),
Next MCAT testing date,
Add Medical Schools and change existing Program type (deadlines, fees, and restrictions apply), and
Release application information to your pre-health advisor.