sometime we may face problem to change password of sa using sql server management studio. that becasue by default this is not set to SQL server and windows authentication mode. Here the solution to do it step by step.
1.Open the SQL Server express management studio
2.Connect to SQL Server using windows authentication
3.Right click the server name and choose properties
4.Go to security tab. Change server authentication to “SQL Server and Windows Authentication mode”
5.Click OK and restart SQL Server
6.Go to SQL Server studio management express
7.Expand the server and choose security and expand logins
8.Right click on SA, from properties modify the password and confirm password
This changed pass should work.