Why I am unable to change the fiscal year end of company? please help me

Hi, everyone

I set up a brand new company and make a mistake to setup the fiscal year end to Sep. Actually I want the year end to Nov. And then, I go to Accounting and Setting---Advanced----First month of fiscal year--- change the month to Dec

After I done all, the fiscal year end under the tab "books to tax- Workpapers " , It still show the year end is Sep, it doesn't work.

Then, I tried the second method to purge the company, wipe all company data and start over . But this method is only wipe all accounting data, not wipe the setting of company, it is not make the company go back to the pre setting status, the year end is still not change.

What should I do now? 

Thanks a lot.