Summarize:

You can easily rearrange the photos inside of your album using the mobile app or at http://photos.google.com. To rearrange photos that aren’t in an album, see Rearranging Photos by Date and Time. It’s at the bottom of the mobile app, and on the left side of the website. The icon looks like a square with a bookmark at its upper-right edge. Once clicked or tapped, a list of your albums will appear. The contents of the album will appear. It’s at the top right corner of both the website and the mobile app. ” The album is now in edit mode, and you can tell this by the editing symbols at the top right corner of the screen. You can drag it as far up or down as you wish. When you’ve found a good spot for the photo, let go of the mouse (or lift your finger from the screen) to drop it there. You can drag as many photos as you wish, but you’ll have to drag them all individually. The photos are now in the order you selected.
Open Google Photos. Click or tap the Albums icon. Select an album to manage. Click or tap the ⁝ menu. Select “Edit Album. Drag a photo to move it. Click or tap the check mark to save.