If you want to change main root FTP Account then you need to change the password of your cpanel Account. However, if you want to change the password of a specific FTP Account, you should follow this instruction.
1. Log into your cpanel account.
2. In the "Files" section, click on "FTP Accounts" Icon.
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3. Under the "FTP Accounts" list, click on "Change Password" or use the "Password Generator". 
"Change Password"
Now you can access your FTP user account with your new password.