In the following example, a database user is associated with a new SQL Server
login. Database user MB-Sales, which at first is mapped to another login,
is remapped to login MaryB.
--Create the new login.
CREATE LOGIN MaryB WITH PASSWORD = '982734snfdHHkjj3';
--Map database user MB-Sales to login MaryB.
EXEC sp_change_users_login 'Update_One', 'MB-Sales', 'MaryB';