1. Open the Keychain Access app. The best way to do this is by using Spotlight.
   
   
   
   
2. If prompted to manage your passwords in the Passwords app, click Open Keychain Access.
   
   
   
   
3. If prompted for authentication, use Touch ID or type your user account password.
   
   
   
   
   
   
4. In your Mac's menu bar, click the Keychain Access menu. Then, choose Settings.
   
   
   
   
5. Click Reset Default Keychains.
   
   
   
   
6. If prompted for authentication, use Touch ID or type your user account password.
   
   
   
   
   
   
7. Type your user account password and click OK.
   
   
   
   
8. Click OK.
   
   
   
   
9. Close the settings window.
   
   

Outcome

A new, blank login keychain is created.

For additional details, please visit Apple's support article "If you need to update your keychain password on Mac".