To change the album cover, go to「Me」>「My Posts」>「My Moments」and tap the cover image to change it as instructed. Alternatively, you can change the cover via「Discover」>「Moments」. Tap your current cover to change it.
Notes:
1. You can only change the cover, but not deleting it.
2. Deleting posts will not affect your cover image.
3. The recommended pixel size for an album cover is 640 x 640.