To change your DOB in the Apple Health app, first open the Contacts app (or go to the Contacts tab of the Phone app), click on your name at the top, then "Edit" and edit your DOB.
Once that's done, head back to the Apple Health app and make sure you are in the Summary or Browse tabs. Click your profile photo at the top right corner of the screen and click "Health Details". If your DOB is not already correct, click "Edit" at the top right corner of your screen, edit your DOB and save it.
Lastly, check that your DOB is still correct for your Apple ID by going to Settings>Your Name>Name, Phone Numbers, Email>Birthday. If your birthday is correct, leave it as is. If it is incorrect, just click anywhere on the "Birthday" option to edit and save your DOB.
Hope this helps.