Probably you use some kind of database in which it is all stored.
The database entries are modified using the update statement. update students set data = '12 / 22/2012 'where data '12 / 21/2012'
where students is the table name, data is the name of the field in the table.
Usually, import java.sql mechanisms are used for this. At the request of "mysql + java Example", google produces a lot of things. Naturally, if you do not have MySQL - look for your own.
Zagogulin here a few. The first. You most likely already have functions to connect to the database, or maybe to execute queries - search. The second. Date formats for different databases may differ slightly. Pay attention!
UNDER ANY CIRCUMSTANCES DO NOT work with a live database immediately. First everything is on the copy !!
And only when it turns out, and you will understand what you have done, and everyone will say that “nothing has been broken”, something can be done somewhere.