Hi,
You may find this post relevant:
https://forums.screenly.io/t/admin-panel-password-reset-screenly-ose/471
To do this, you will need to log in to the Pi itself, but not Screenly.
You can press CTRL+ALT+F1 to access the command line on the pi.
The default username for the pi is “pi” and the default password is “raspberry”.
If your predecessor has not changed this password then hopefully you can use the above instructions to regain access to Screenly.