To change the authentication mode from Windows NT Authentication Mode (only) to Mixed Mode, use the following steps: 

  1. Open the Enterprise Manager. 
  2. Expand a Server group. 
  3. Right-click a server name, and then click Properties. 
  4. Click the Security tab. 
  5. Under Authentication, click the SQL Server and Windows option button. 
  6. Restart SQL Server in order for the change to take effect.

For more information, see this