If the admin password has been changed, and nobody can remember what it is, the NVR must be returned to us in West Melbourne for hardware/firmware reset. It's not possible to do a password reset remotely, so be very careful when changing your password!
A recommendation to avoid this is for the installer to create an "everyday" login for the clients to use all day, every day, rather than use the admin account for everything. In the "Rights" section of the "Account Parameters">"Account Setting", you can allow the "everyday" account to do almost everything you can do with the "admin" login. Except to change the admin password, of course!
And if you do change the admin password (which is still a good idea from a security point of view), please keep a record of this in a safe place. That could be using a password manager app on your phones or computers, a physical copy taped to the back of a disused fridge in the storeroom, or in a locked briefcase chained to your wrist, or something more sensible, so long as you know where that is.