After creating the account, you can use that account. It will be like a new Mac. There is also another way to reset the password ***** old existing account.
If the above doesn't work, you can follow the steps below to reset the master password ***** existing account.
Turn off the 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 X Utilities” menu.
Click on the “Utilities” menu in top and select “Terminal".
Type (without space) : resetpassword
Press return. Confirm the account and then change password ***** reboot as normal. Login with new password.
If you did everything right, you will be able to login to the existing account and you will get a keychain error. That is normal. Let me know how it goes.
Remember, this is not my final answer. If my solutions didn't work please don't rate me bad or poor. Instead please reply back to me and I will continue to help you until we fix the problem or you are satisfied.