If you are currently connected to a database, you must first disconnect. You can find the exact instructions here .
Select the entry "Continue and manage database connection".

To connect to an existing PaperOffice database or to an existing backup, go to your PaperOffice login window and click Continue. This window appears:

Select "Start database manager".

In the next step, select "Local database" and click on the corresponding icon:

Then select "Use existing database or restore from backup". In the next step in the left section "Default" click in the first field and select the path to the existing database. Confirm your entry by clicking on "Check information, save and continue".

In the next step, select the MasterKeys for the database you just selected and click "Check and continue." The MasterKeys will be validated in the next step and you will be connected to your database.

The connection to your database proves to be successful if the message "Connection successfully established" appears in the upper right window.

This completes the connection to your existing database.