How do I forget or change the username and password for file sharing?
Go to the Finder’s Go menu and choose Utilities
Select Keychain Acces
Select the Login Keychain then select the network password or internet password for the printers and/or shares.
Hold down the “control” key and click the keychain entry. Select Delete.


Now, access your network resource (printer or share) and re-enter your new username and password when prompted.