In order to change the authentication for an open database, click on Properties (CTRL + i) > General > Change. This will start an assistant where you can change the authentication.
First you have to enter your current credentials and click on Next in order to authenticate.
After that you can select another authentication method or just define a new master password for the file.
Authentication by: Here you can choose one of several methods of protection for your database:
Master password - The classic and (depending on the complexity of the master password) safe method of encrypting a database using a master password.
Master password and key file - The database is protected by means of a master password and a key file.
Key file - The database is protected by means of a key file. Key files contain complex encryption keys which are very safe and cannot even be cracked by a brute-force attack. However, please remember that anyone who has access to your key file and your database can read all your passwords! You should therefore treat your key file like a real "vault key" and always store it in a safe place (e. g. on a USB stick). Use the button Generate key file to create such a key file.
NOTE: If you use a hint for your authentication, make sure to update it as well. Otherwise you might get confused when you need it.