Step 3: Enter the following coding into the text box:
DELETEFROMdnames_changeWHEREdname_member_id = x
x = Member ID - You may find this in the URL of a users profile.
Eg: If the username was 'Plum' and it was the 4th account to sign up to my website then the following would be it's profile ID: http://www.<WEBSITE>.com/user/4-Plum/
To delete all display name history from every user type in:
Step 4: Hit 'Run Query'
Step 5: If done correctly it should say "Query executed successfully"
I do not take any credit for the coding provided in this tutorial.