Open the post you would like to change, then tap the ••• menu in the upper-right corner.
Select Edit to open the Edit Post page.
From there, you can:
For a video, tap Edit cover on its thumbnail to choose which frame appears on your timeline.
When you are finished, tap Save changes.
Open the post, tap the ••• menu in the upper-right corner, then select Delete Post.
Follow the confirmation prompt to remove the post from your timeline.