Please help us to help you by reading and following the posting guidelines located in this thread: Please read before requesting assistance.  The more information you give us the better we will be able to assist you. Thank you.There is currently no way to do this using the UI, you would have to modify the source code, or click the edit button in the upper right corner to edit the ticket details.

/agrees with @[deleted]

It is possible to do.  You would have to make a PHP query to the database to pull all help topics and display the options in a select list.  You would also want to check each topic to see if it is the one that is currently assigned to the ticket, so that you can set that as the selected topic.  You would also need a save button/form and a post, or a jquery/ajax statement for an onchange event.However, without following the instructions blueyeguy linked, we aren't able to provide you with more information as each version of osTicket does things a little differently.