If you do not have access to WHM then cPanel password for a particular account can be updated from shell with root access.
The following steps should help you to change / reset / update cPanel password of particular account from shell:
1) Access the server as a root user through SSH
2) Execute following command
/scripts/chpass username password
username : cPanel account username
password : new password you wish to set
Note: Password should not contain special characters
3) Synchronize the new password with the default FTP user
If any feedback, queries are always welcome!