While lychee has a UI to change the password, there is no UI to reset the password. If you forget the password, reset the password of the admin user, use the Web terminal.
# sudo -E -u www-data php artisan lychee:reset_admin
After running the above command, just logout from Lychee and it will ask for creating the admin user again. Note that existing data is not lost when resetting admin credentials.
Using an existing folder structure
Lychee has it's own folder-structure and database. You have to re-upload or re-import all your photos to use them.