Change LogOn properties of a SQL server instance

Change LogOn properties of a SQL server instance

I want to change the logon properties of a sql server 2008 r2 instance by
using code. Manually i can do it in SQL server configuration manager by
right clicking on instance name->Log On. Under Built-in account change
Network service to Local service. Then click on apply. SQL instance will
be restarted to effect the changes.
Now hoe can i do the same(Network service to Local service) using code .I
think it can be done using WMI. So please help me on this.