There are several options at the top of each discussion that allows you to search, sort and change the view of the posts.

Search entities or authors - This field allows you to filter posts based on text or the name of the author. To use this, start typing text into the search field. The post/replies list below the search field will automatically start to filter posts based on the text you have entered.
View - This dropdown filter allows you to select between all posts and read/unread posts/
Sort - The sort filter allows you to control if the discussion shows the newest or oldest posts at the top. When the sort arrow points up then the oldest post will show first. When it points down the newest posts will show first.
View Split Screen - Enables you to view threaded replies in a split screen sidebar on the right. Press this button again to return the threaded replies to the inline discussion view.
Expand Threads - Displays all threaded replies once. To collapse the threaded replies, click the Collapse Threads button.
Replies and unread - The total replies and unread replies are shown at the top of each discussion. The number on the left indicates total replies, while the number on the right indicates the number of unread replies.
Subscribe - If you want to be notified when replies/posts are made to a discussion then click the Subscribe button.You are automatically subscribed to any discussions you reply to. If you are subscribed, the icon displays a solid green background. To unsubscribe, click the Subscribe icon again. You will need to ensure that you are set to receive notifications on discussion posts to receive the notifications.

Canvas guides

How do I view and sort discussion replies in Discussions Redesign as an instructor?

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