Okay, I have managed to push my site to the first position when searching in google.com.vn with keyword "may chu ao Linux". I keep that for 1.5 month. At that time, it's the index file of my site.
Now, I have moved that page to inner page, and put different content in the index file. The result is my position disappeared.
How to keep the position if I want to update index file content once few months? Each time changing, I want to put different content for different service, so the keywords should be different.