If you are looking for an option to set one webmail client as default please follow the steps to disable the other two email clients since cPanel by default contains 3 webmail clients Roundcube/Horde/SquirrelMail as default.
In order to disable a webmail client follow the steps below:
1.Login to WHM
2. Select tweak settings Home »Server Configuration »Tweak Settings
3. You would get the option to disable Webmail client under 'Mail' in Tweak Settings
