If you remember your previous password
1. When prompted to enter/unlock/sync your keychain, enter your previous password.
2. Click Update.
If you don't remember your previous password
Note: These steps will reset your keychain and any saved passwords will need to be re-entered.
1. Ignore the popup by clicking Cancel.
2. When you are logged into your account, open Keychain Access. You can easily find it by opening the search bar in the upper-right corner and searching for "keychain".
3. Right-click on Login in the left pane.
4. Choose Delete Keychain.
5. In the window that appears click Delete References and Files.
6. Restart the computer.