Log in to webmail using the URL: http://www.YourDomain.com/webmail/ or http://www.YourDomain.com:2095
Linux Web Mail comes with 3 types of Webmail Interfaces for you to choose from:
Horde
SquirrelMail
Roundcube
Select the interface you want to use and enable AutoLoad. (For example here is SquirrelMail)
Once you enable autoload for SquirrelMail, it will be the default interface for every time you login in your Webmail.