If you want to change the order of your pinned posts, do these steps:
1. Click the pinned post you want to put at the top, so that you are now in the enlarged view of that post
2. Click ⋯ and then "Unpin"
3. Wait a little bit
4. Click ⋯ and then "Pin"
It's important you click to enlarge the post before you unpin it, because otherwise it will disappear back to its original place on your timeline and it may be fiddly to find it again (especially if it's an older post).