Q: Hand me down iphone

So your boss doesn't remember his iCloud account password? That is what I gather from what you are saying. If that's the case, have him reset his password. If this was someone else's phone that works with you, have the boss get the password. If you can not sign out of this iCloud account and you need to restore or update the device, you will not be able to activate it again. It will be useless to you.

If you forgot your Apple ID password - Apple Support

You need to contact your boss and ask him to remove this iPhone from his list of devices. Otherwise, your iPhone will be a useless brick.

Find My iPhone Activation Lock: Removing a device from a previous owner’s account - Apple Support