You are able to modify the periods within each fiscal year in the fiscal calendar. When you change the end date of one period, the start date of the subsequent period is updated automatically to the next day after the end of the previous period. Each period must be at least one day in length.
However, changing the fiscal year end will affect financial reports that report by period or that compare the fiscal periods from one fiscal year to a prior year. These reports may not be accurate when they report on periods that are not a full month or if period 5 (for example) is May in one fiscal year, but July in another fiscal year.
There are 3rd party products available which help you to modify the fiscal year, and then updates all the transaction details into the new periods created by the new fiscal year.