I have installed my WordPress blog site here, http://www.test.com.au/demo, now I want to change my root path to this http://www.test.com.au, but when I transfer all files to here, I can't really access the site correctly.
I don't know what is actual method to do this.
You may need to do more than just transfer files. You will need to change a few database settings as well in wp_options. Do a select * from wp_options and see which settings are still pointing to your old location, then you can change those.