Go to the Activision Website
Visit: https://www.activision.com
Log in with your Activision account
Access Account Settings
Click your profile icon > Account Settings
Go to the Basic Info section
Change Your Activision ID
Click the edit icon next to your Activision ID
Enter your new display name
Click Save
You get one free name change token every 6 months
If you don't see the option to change your name, you may be out of tokens
Your new name will appear in Warzone, MW, Cold War, MWII, and MWIII
You can also change it through the game:
Open the game
Go to Settings > Account & Network > Activision Account
Select Change Display Name