Usually configs are copied after the install into the editable location:
/var/snap/photoprism/current/config
But they will be replaced after the next version upgrade if you edit them.
Can you tell me which config you want to change?
If it is the originals difectory then it points here currently:
/data/photoprism/photos/originals
So what I do to add more is just link other dirs like my nextcloud files:
cd /data/photoprism/photos/originals
ln -s nextcloud /data/nextcloud/[user]/files
Strange that photoprism devs did not allow adding more dirs from the UI.