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
The lack of a "Pin" option on the official apps is yet another reason to switch to an alternative:
#FediTips

How do I pin posts on my Mastodon profile? | Fedi.Tips – An Unofficial Guide to Mastodon and the Fediverse
An unofficial guide to using Mastodon and the Fediverse

Which Mastodon apps can I use? Should I use the official app or a third party app? Are there other options? | Fedi.Tips – An Unofficial Guide to Mastodon and the Fediverse
An unofficial guide to using Mastodon and the Fediverse
