Just click the new "edit" button when opening an album and reorder photos using drag and drop. You can also add photos from other albums.
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Open a photo, click the info icon and then click the edit button next to the date and time if you want to change them.
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Another recently added feature: changing the album cover. Open a photo from an album, click the 3-dot menu icon and select "use as album cover".
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{ via +GooglePhotos }