Hi MichelleLee25,
I tried the method provided by Charles and it worked perfectly.
Besides, you may also try the steps below to change the heading format:
1. Locate your cursor in one of the words with Heading 2 style, go to Home > Editing > Select > Select Text with Similar Formatting:

2. With all Heading 2 text selected, please go to the Home ribbon, click Heading 3 button in the Styles group:

Hope you can find this helpful.
Best regards,
Yuki Sun
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