OK, I wait for whole afternoon to search the answer, but nobody drop me one word as tips, so I have to work hard to find the answer by myself.  and finally I got it. OK, now I will show you how to resolve the problem:
1. go ahead(anyway) if you entered a wrong category at '+New Category' prompt, don't worry about the wrong name, leave it there and write down some words in blog body(the bigest text-area in that page) and save/submit
2. Click link 'Topic' in the navigator-bar on the left side of the page, you will see your all blogs in the main-frame of the page
3. Certainly, you can see the article which you posted just now, and for now the article is under a wrong category, so you can find a PEN sign on the right side of the main-frame of the page
4. Click the PEN sign directly, you will got a pop-up message box, and the category name will be fully selected and under editable state,  enter any new name into that field and click OK

Congratulations! you get the category rename successfully now.