If you need to change your Namecheap reseller account password or any cpanel reseller account password, follow these steps:

1) find out what is Your WHM or cPanel URL address

It might be
WHM: http://host14.registrar-servers.com:2086/
cPanel: https://host14.registrar-servers.com:2083/

Basically the URL is the hostname of the server you are hosted on and 2086 is WHM port, 2083 is cPanel port.

2) Use your cPanel URL to login your reseller account cPanel

Finally when loged in, on the top, search for icon named "Change Password". Change your Namecheap or other reseller account password.
b) If you lost your cpanel reseller login details and your hosting welcome email is lost too, contact your hosting provider to reset your password and send you your username. It will be sent via email probably.

Your pasword will be changed both for your cpanel and whm account.