A knowledge base article about How can I organize my bCourses discussion forums? provided by the UC Berkeley IT Service Hub - Knowledge Portal
By default, discussion forums are arranged by recent activity. If you would like to manually order the forums to a specific order, you will have to first pin the forum and then you will be able to click-and-drag them in the order you prefer.
Step-by-step instructions are available in this Canvas Guide: How do I pin a discussion in a course?