Look inside WHM and Cpanel !
hi,
IF its reseller a/c you can use the "Password modification" from WHM to change the Login details, or login to yourmasterdomainhere.com/cpanel/ and change password, changing cpanel pass would also change your WHM password.
If you are root user on the server you can find the "Change Root Password" option near the top of left navigation links.
see ya,
mohit