To change master password of cPanel, you must do this from your customer area on :
Click on your relevant service
Then, left column, click on 'Change password'
this will also change the main FTP password, update it in your FTP software without delay to avoid blocking
For reseller accounts :
Same, only for your reseller account.
For your subaccounts, it is from your WHM access / list accounts / click on the small cross of the account whose password you want to change