You can easily change the cover image on your album.

Web App

1. Open the album in question.
2. Hover over the photo you would like to add as the cover and select the pencil (edit) icon.
   
3. Select Cover Image option. 

Android and iOS

1. ​Open your album.
2. Long press the photo you want to make your new cover until the alert pops confirming you want to make it your new cover.