In the Library folder, you’ll find a folder named Keychains. Drag System.keychain and any other files in the Keychains folder to the trash. Restart your Mac, and login with your new password.
“But wait!” You cry. “I’m getting popups asking for my password and neither my new one or my old one will work!”
Fortunately, there’s a quick way to fix that. Locate the Keychain Access application, which is housed in the Utilities folder. Along the top bar, click Keychain Access, then Preferences. Click Reset My Default Keychain. Now navigate back to the Library folder and delete any files in the Keychains folder.
Log out and in again, and the popups are gone!
For more information and step-by-step screenshots, visit our Knowledgebase article.