You can set it at the horde install path:

Path will be

#/usr/local/cpanel/base/horde/imp/config/

Files to edit.

conf.php
config.xml

Path below is horde base path

#/usr/local/cpanel/base/horde/

To increase the memory you will have to increase horde php conf below

# /usr/local/cpanel/3rdparty/etc/hordephp.ini