If it is Mac OS X 10.7 or later, then you can do the following steps to reset the password. If it is lower than that, let me know and I will send another way to reset it.
You can reset the password ***** booting into Mac OS Utilities screen. First, go to System Preferences select iCloud. Scroll down and if the Keychain option is enabled, then turn it off or uncheck it. Ignore it if you don't see this step.
Then follow the steps below to reset the password.
Turn off your Mac.
Turn it back on and immediately hold down Command and R keys at same time for 10 seconds.
This will boot your Mac to "Mac OS Utilities” menu.
Click on the “Utilities” menu in top bar and select “Terminal".
Type (without space) --> resetpassword
Press return. This will bring Reset Password ***** Select the hard disk at the top column, select the user account from the drop-down under it, confirm the user account and then change the password ***** Save. Then click on the Apple icon and select Shutdown.
Turn it back on. After restarting, if you get any message about Keychain then click Create New Keychain.
If you are still getting keychain error, then reset the keychain access database. Follow the steps below to reset Keychain;
In Finder, hold down Option key on your keyboard, click on Go menu at the top bar and select Library. This will open Library folder. Trash Keychain folder in there and restart your Mac.
Please let me know if that didn't work, so we can continue troubleshooting.
Note : This is not my final answer. I strive to provide Excellent service or 5 star service. If you feel you've received anything less or if you are not satisfied or if my solutions didn't work, please don't rate me bad or poor or ok. Instead, please let me know and I will try my best to help you until you are fully satisfied or the issue is resolved