You need a database administration tool, e.g. www.phpmyadmin.net
(might be pre-installed by your hoster; if you install it yourself, make sure to restrict access to the phpmyadmin directory)
phpmyadmin --> phpbb_config --> browse --> edit --> board_startdate
if you want to change the registration date of the admin account:
phpmyamdin -->phpbb_users --> browse --> edit your account --> user_regdate
that number is the number of seconds since the 1st january, 1970.
backup your database before changing anything.