When using cPanel webmail you're able to change this directly through the cPanel interface like so...

However, when set to CrossBox, webmail will automatically redirect you to our CrossBox server where you're unable to select which email client you'd prefer to use.

The fix for this is to edit the default webmail client within cPanel to point back to Roundcube or horde - this can be done within the cPanel file manager and the file is found at /.cpanel/nvdata/email@domain.tld_default_webmail_app

This file can simply be removed completely to reset the default webmail login and enable you to switch between mail clients again.

Additionally, if the above steps don't work this may be cached within the cPanel account, this can be removed by deleting the .cpanel/nvdata.cache in a similar fashion to the above.