If you created the topic and it's within the one-week edit limit, go to the post you created the topic with. Click on three dots in the upper right-hand corner and click "edit" in the drop down menu (For visual guidance, see here). As it's the post that created the topic, the title can be edited. Once you're done, click "edit topic" and you're done. 

If the one-week edit limit has passed or you aren't the topic creator: Please report the post via the report feature (attention icon) to request a change to a topic title. (For visual guidance, see here).