By default, a popular post is defined as one that has at least 15 replies, 200 views, and had a new post made within the last three days.
To change the popularity settings:
Navigate to Control Panel Dashboard > Forum Administration > Configuration > Global Forum Settings > Popular Posts.
Specify in Popular Post Replies the number of replies that are required for a forum thread to be categorized as popular.
Specify in Popular Posts Views the number of views that are required for a forum thread to be categorized as popular. This view is a total of views for all posts within the thread.
In Popular Posts Age, specify the number of days within which a thread must have a new reply to be included in the popular thread list.
Click Save. It may take up to 15 minutes for your changes to take effect across the community.
The popular post criteria selected in the previous procedure drive the number of replies, views, and age of posts that constitute popularity when you select the Show only popular threads check box in the Forum - Threads widget (pre-5.6) or Forum - Thread List widget (5.6):
Selecting this check box applies the criteria to the contents of the widget.
To learn about creating a list of most popular blog posts, see How to: List most popular blog posts.