Dear @Kishore Mani,
Thank you for posting your question in the Microsoft Q&A forum.
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As you mentioned that you need to change your legal name before the AZ900 exam. So could you please verify for us that you need to change your legal name in profile to do examination? if it's correct, you can click to your profile picture on the left above corner, then choose setting. You can see your information in there and can update your legal name as attached screenshot below:
Besides that, for more detail information you also can refer to this article Credentials support to update your legal name. You can contact to your IT/admin of your school so they can help you raise support ticket for you.
I hope information above helpful, if you have any other questions, please feel free to reach out.
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