Sure, right here
The MODX_CORE_PATH entry needs to be changed in core/config/config.inc.php -- *after you copy the core to its new location*
And the core path also needs to be changed in the three config.core.php files in these three directories:
modx root
manager
connectors
That should do it. Just copy the core directory to its new location and change the paths listed above. Back up each of the 4 config files by duplicating them and adding "old-" in front of their names before editing the non-old ones. Rename the original core directory so you know it's no longer in use. That way you can revert easily if it doesn't work.
After making the changes, delete all files in the core/cache directory and clear the browser cache and cookies before logging in.
To help you understand -- whenever MODX needs to know where things are, it reads the nearest config.core.php file to find out where the core is and what the
config key is (determines the prefix of the main config file; default is 'config'). Now it knows the location of the core and the name of the config file. It reads that file (usually called config.inc.php) to get all the locations.