The sa user or any user in the admin group can set his/her own password, or can set a password for any other existing user as follows:  

• Click on Tools \ Users

    

The User admin dialog box will then appear with a list of all current users.

    

• Select the user for whom you are setting the password by clicking on the user. 
  The Selected User dialog box will then appear. 

    

• Type in the password for the user in the Password field.
  
  
• Click on update_password.php in the Plugins box if you wish the user to have permission to update his/her own password.  This is optional.  To learn about how a user can update his/her own password, click on the updating your password.
  
  
• Click Update.

The message 'user information updated' will be returned.  The user will then need the new password in order to logon.    

A user's password can also be set on creating a new user.