You can pin posts to your Mastodon profile if you are using Mastodon's web interface, Mastodon's web app or a third party Mastodon app:
1. Go to one of your posts which you want to pin
2. Click ⋯ on that post
3. Select "Pin"
You can unpin a post by repeating this but selecting "Unpin".
Lots more info about pinning posts on Mastodon at https://fedi.tips/how-do-i-pin-my-posts-on-my-mastodon-profile
The lack of a "Pin" option on the official apps is yet another reason to switch to an alternative: https://fedi.tips/which-apps-can-i-use-should-i-use-the-official-app-or-a-third-party-app
