If you know your old keychain password

1. In Finder, click Go and select Utilities.
2. Double-click Keychain Access.
3. Select login in the upper left side of Keychain Access.
4. Click the Edit menu and select Change Password for Keychain "login".
5. Enter your old password for your user account in the Current Password field. This is the password you were using before your password was last reset.
6. Enter your new password of your user account in the New Password field. This is the password you're now using to log in to your Mac.
7. Enter the same password in the Verify field.
8. Click OK and restart your Mac.

If you have forgotten keychain password

Mac OS 10.11 "El Capitan" and above

1. In Finder, click Go and select Utilities.
2. Double-click Keychain Access.
3. Click the Keychain Access menu and select Preferences.
4. On the General tab, click Reset My Default Keychain.
5. Enter your current username and password.
6. Restart your Mac.

Mac OS 10.10 "Yosemite" and earlier

1. In Finder, click Go and select Utilities.
2. Double-click Keychain Access.
3. Click the Keychain Access menu and select Preferences.
4. On the General tab, click Reset My Default Keychain.
5. Enter your current username and password to secure the Keychain.
6. Click the First Aid tab.
7. Ensure that Synchronize login keychain password with account is ticked.
8. Restart your Mac.