1) Log into webmail
From the Internet Explorer address/URL bar, key in "http://webmail.yourdomainname.com" and press enter. (Eg: if your domain is 123.com, your Webmail access address is "http://webmail.123.com")

2) Go to Webmail Options
When logged in, click on "Options" at the top menu.

3) Select "login" menu, change password here.
Select "login" at the options menu screen. You need to enter your current password in the "current password" field. Then enter a new password and confirm New password field. Click "update" and you're done!

NOTE:
Ask your Company's Web Administrator if you lost your password. He/ She can re-set your email password.