SQL Server 2012 Service Master Key Regenerate
In order to regenerate your master key, you need to open it with the old password.  Then, once it's open, you can use the REGENERATE syntax to change it to a new password:

OPEN MASTER KEY DECRYPTION BY PASSWORD = '0ldP@55w0rd';

ALTER MASTER KEY REGENERATE WITH ENCRYPTION BY PASSWORD = 'N3wP@55w0rd';

CLOSE MASTER KEY;


Which is why you should always have your encryption passwords safely secured.  Without them, you could be in a place where you can't decrypt your databases.