Permissions required to alter a database file Totally at the beginning in administering a database.

Using EMS SQL Manager ... latest version.  How do I over come below 

ALTER DATABASE joneslocker2
MODIFY FILE (
    NAME = joneslocker2, 
    MAXSIZE = UNLIMITED )
GO


which results in an error:


  User does not have permission to alter database 'joneslocker2', the
  database does not exist, or the database is not in a state that allows
  access checks.


I don't have a clue what to set next.
You need to GRANT ALTER DATABASE permission to the user.

GRANT ALTER ON DATABASE:: joneslocker2 TO username


Below script will help you to find what permissions are assigned at the database level :

SELECT prin.[name] [User], sec.state_desc + ' ' + sec.permission_name [Permission] 
FROM [sys].[database_permissions] sec 
  JOIN [sys].[database_principals] prin 
    ON sec.[grantee_principal_id] = prin.[principal_id] 
WHERE sec.class = 0 
ORDER BY [User], [Permission];


Check my answer here for an example on why you are getting the error.

Refer to : Database Engine Permission Basics