How to Change the Default Database for a Login in SQL Server

To change the default database for the login to a different database:

USE [master]
GO
ALTER LOGIN [John] WITH DEFAULT_DATABASE = master
GO

change_default_database_for_login

To change the default database for a login using SQL Server Management Studio (SSMS):
Right click on a login, click 'Properties', and choose default database:

change_default_database_for_login_ssms