First, go to the album and tap the 3-dot icon on the top-right. This will open the album's options menu. Tap Change album cover:

Go to the album, then tap the album options menu, and select the Change album cover option Go to the album, then tap the album options menu, and select the Change album cover option

Now it’s time to select the photo that will replace the album's cover image. Tap the photo you’d like to set as the cover for that album. The change will be immediate, and you’ll be able to see the new cover appear in the album screen and My albums.

Left: tap the photo you would like to make the cover of that album. Right: view the new cover appear in My albums screen. Left: tap the photo you would like to make the cover of that album. Right: view the new cover appear in My albums screen.

And that's all there is to it!