Most security experts refer to enhancing SQL Server password security by changing password. To change SQL password is easy and one can make it through the following methods:

 

Method 1: Changing SQL Server user password by using MSSQL Server Management Studio Express

1. Login to your Server using MSSQL Server Management Studio Express. Make sure to specify the correct SQL server for your database. 
2. Expand the tree menu on the right by clicking on the "+" next to Security followed by clicking on the "+" next to Logins. Your users for the database in question should be listed.
3. Right click on the user you wish to change the password for and choose the Properties option at the very bottom of the menu.
4. Under the general section of the window you will see the option to type in and confirm a new password. Once the new password has been typed in, then click the "Ok" button in the lower right to finish.

 

Method 2: Altering SQL password by using sp_password

Any user can change his or her SQL user password with sp_password. However, only a System Security Officer who belongs to the Server’s security administrator group can execute sp_password to change another user’s password. Now please first log into SQL Server with Windows authentication mode, then open a new query window and next type the below commands to run sp_password to change password by pressing F5:
sp_password [ [ @old = ] 'old_password' , ]
{ [ @new =] 'new_password' }
[ , [ @loginame = ] 'login' ]

 

 

Method 3: Change forgotten SQL Server password with SQL Password Recovery

It’s easy to change SQL password when you have access to the database. However, what if you lose access to the Server due to SQL Server password is forgotten or lost, how would you change your SQL Server password as easy as method 1 and 2? And is there any chance? Well, a professional SQL password recovery tool can tell you everything. Now please follow me to recover the forgotten SQL password with SQL Password Recovery:
Get SQL Password Recovery here.
Step1: After download the tool, then quickly install and launch it on a computer. Next in the pop-up interface of the tool, click “Open” to load target master *.mdf file.
Step2: After open the *.mdf file, all user names with their passwords will be displayed in a list.  Now just select the user account whose password needs to be changed and then click “Change Password”.
Step3: Now enter a new password for this account and then confirm it with clicking “OK” button to change SQL Server password process.

 

Source: http://www.sqlpassword-recovery.com/change-sql-server-password.html