SQL Query
update ubbt_FORUMS SET FORUM_DEFAULT_TOPIC_AGE=0

will do it for all forums (just paste the query into your database tools section)

change 0 to xxx for number of days too..

example: to set all forums to 2 weeks:

SQL Query
update ubbt_FORUMS SET FORUM_DEFAULT_TOPIC_AGE=14

*assumes your table prefix is 'ubbt_' *