You can't.

Review the documentation at https://doc.openerp.com/7.0/book/3/3_9/entries_at_the_start_of_a_year/#closing-a-financial-year

When the year is closed, you can no longer create or modify any transactions in that year. So you should always make a backup of the database before closing the fiscal year. Closing a year is not mandatory, and you could easily do that sometime in the following year, when your accounts are finally sent to the statutory authorities, and no further modifications are permitted.

I don't recommend closing a year because it is irreversible.