EdReady has no built-in ability to retake the initial diagnostic because it is not a high-stakes exam. The initial diagnostic test is designed to find out what you already know and provide you with a personalized study path so you don't have to study and test again on concepts you are already familiar with.
Most students are supposed to keep working through their study path until they reach the target score set by their school. In general, the entire study path in EdReady "is" the test. Achieving your target score is considered to be the final test result and it indicates that you are "ready" for whatever your goal is.
The only way to start over is to delete your account or create a new account with a different email address. However, each school that uses EdReady customizes how EdReady works to meet the needs of their particular use case. So your school may have different requirements or protocols, and they may even have an option for you to do a "practice" run through EdReady, then take a "real" test later.
EdReady support staff cannot take any action without the permission of your school's administration. If you think you need to start over in EdReady, please consult with your instructor, advisor, or placement testing department (whoever told you to use EdReady) to find out how to proceed.