The most simple way is to update the database with SQL commands like:
update minibbtable_topics set forum_id=CURRENT_FORUM_ID where forum_id=MERGING_FORUM_ID;
update minibbtable_posts set forum_id=CURRENT_FORUM_ID where forum_id=MERGING_FORUM_ID;
For example, if you have a forum A, and you want to move all topics to forum B, then A will stand for MERGING_FORUM_ID and B is for CURRENT_FORUM_ID.