How to reset password after login

Change your user account password with the System Preferences app.

System Preferences > Users and Groups > Select your User Account > Click on Change Password

I have tried this approach several times, and it won't accept my existing password when I try to make the change. I had a total breakdown in my email today, spent a load of time talking to Apple folks -who were kind enough to speak to me on an iCloud email problem despite the fact that my warranty had run out. They couldn't solve the email problem and essentially told me to wait a while, and then just hung up on me. The password protection I had worked until after I let the Apple folks use their access to my account online. Now, I can't change the password.