Local Photos Store is the folder, where Phototheca copies photos from memory cards and smartphones to. More details are here: Overview of importing
To change the location of the Local Photos Store, follow the next steps:
1. Go to menu File->Manage Library… and the dialog to manage a library appears
2. Click the “Change” button next to the text box with a Local Photos Store location
3. Browse to a new location for Local Photos Store
4. Confirm change
NOTE: Phototheca would not copy or move existing photos from the old Local Photos Store to a new location. If you would move photos from the old location to a new location manually, then Phototheca loses the track of files and you would need to re-import them from a new location again.