On your Blog Settings tab select whichever blog you want then scroll down to "Default display options" and choose the skin of your dreams - and a few other options like if the visitor can select the skin of their dreams.

BTW when you download a skin you want and unzip it make sure you have a bunch of files (and maybe folders) in the unzipped folder. NOT a folder with the same name that then contains a bunch of files (and maybe folders). For example if you downloaded "foobar" skin and unzipped it you should have a "foobar" folder that has files in it instead of a "foobar" folder that has a "foobar" folder in it.

Thks. I go to the setting tap, I can't find "Default display options" I check my folder. I actually installing the skin into

skins/newskin

newskin is a folder that I upload all the file into it. It doesn't have any folder. I hope you can login to check for it
http://blogyourownbusiness/admin
username: byob
password:123456