Step-by-Step...

Change Mac Password

Apple menu > System Preferences > Users and Groups

Select your name...

In the lower left corner click the Lock icon and authenticate with your Mac's Local account password.

 
Click 'Change Password...' and put in the password you just entered

Put in your new password, and type it again to verify.

Special care should be given to keep this password protected.

Apple recommends using a hint.  Obviously never put the password itself in the 'hint' area.  

Click Change Password... and you’ve done it. 

Your Mac password has been changed

Remember that your local Mac password is not the same as the password you may use to access network services or other
email accounts you may use.

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