This is how you change a cover photo of an album:
Go to photos.google.com or open the Google Photos app on your mobile device
Choose "Albums" or go here: https://photos.google.com/albums
On mobile: Go to Library and scroll to Albums
Click the album that you want to change
Click the picture you want to use as the new album cover
Click the 3 vertical dots ("More options") in the upper right corner
Click "Use as album cover"