It may be difficult for a newbie. You will need to build SQL commands list manually.
Follow backup file (I suppose it is standard plain text backup) and locate this topic's ID in `topics` table backup. SQL string would look like
INSERT INTO `topics` VALUES (TOPIC_ID,...
where TOPIC_ID stands for the numerical ID. Copy the entire INSERT statement into some new plain text file.
Then follow `posts` table backup. Where it says
INSERT INTO `posts` VALUES...
follow all strings which contain TOPIC_ID as the third value after comma in INSERT statements. Then copy these strings to the new plain text file where you've had topic's statement already.
Then you have to remove TOPIC_ID value from `topics` statement and apply it on your current live forums table. It should insert new topic with its new ID. You have to know this ID and change it in `posts` statements. Then apply all `posts` statements over your live table.