You can easily reset the administrator password ***** booting into Mac OS X Utilities screen. First go to System Preferences select iCloud. Click Sign Out button there and sign out. If asked to delete or keep stuffs, select Keep. If you only see Delete, then click Delete or OK. It is OK, you can reconnect to iCloud later.
Follow the steps below to reset the password.
Turn off your Mac by holding down the Power button for 5-10 seconds.
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 X Utilities” menu.
Click on the“Utilities” menu in top bar and select “Terminal" (look at the top bar of the screen to see Utilities menu)
In Terminal, type(without space)--> resetpassword
Press return. This will bring Reset Password ***** Select the hard disk at top column, select the user account from the drop down under it, confirm the user account and then change 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. Use new password ***** asked.
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 (top of screen) 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.