System users password has expired

Open up cmd.exe and type in SQLPLUS

 Enter user-name: SYSTEM
 Enter password: the expired password 

It will now let you type in a new password for the SYSTEM user

If SQLPLUS is not available, you can also reset an expired password from within SQL Developer:

• Save your connection information (if not done already)
• Right click on your connection
• Click on the “Reset Password” option in the menu, enter old/new password blah blah blah

If someone drops by from a Google search but uses IntelliJ: You can reset the system password in the connection properties' "Advanced" tab. There you can set the new one as a value for the oracle.jdbc.newPassword key.