This is a really nice feature from Facebook to manage the comments better than before and move the unneeded comments at the bottom of the thread. Here is a step by step guide on how you can use this feature on your Instagram app. Before attempting, make sure that your Instagram app is updated to the latest version.
Steps to Pin Comments on Instagram Posts
How to Unpin a Comment
Wrapping up
This is how you can pin or unpin the comments on any of your Instagram posts. This is a really nice feature released by Facebook gives more control over the comments and helps to manage them better.