Just what the title says, I need to change the password for an existing sql server login and I want to do it via sql script.
ALTER LOGIN
http://msdn.microsoft.com/en-us/library/ms189828.aspx
alter login mylogin with password = 'mylogin'
If the user already has the ALTER ANY LOGIN permission (i.e. the user can change any password for any user) then this works:
alter login mylogin with password = 'mylogin'
Otherwise, if you don't want to make every user in your system a superuser, add a parameter of the old password for the same command:
alter login mylogin with password = 'mylogin' old_password='oldpassword'