There are several ways to change SA password,by using SQL Server Management Studio, SQL script or third-party software. Check out this tutorial for detailed steps. I hope this will be helpful for you.
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I follow these two into one section because the commands for
them are entered at the same place.
1. osql -S PC_NAME\SQLEXPRESS -E
Be sure to change PC_NAME to whatever your
PC name is or you will get a big long message saying you messed up.
8. At the
next prompts enter the following commands:
1> alter login sa enable
1> sp_password NULL,'new_password','sa'
the “SQL Server (SQLEXPRESS)” service
the “-m” from the Start parameters field
At this point you should be able to login to SSMS using the
sa user account and the new password you gave it.
These steps will help you to change your SQL server express