No but you can on the Mac, which updates it on the phone. I use iCloud photo library and it updates immediately on the phone. Go to the album on your Mac, right click any photo and choose, "set key photo".
You can lightly tap a photo and move it to the first in the album and that will change the key photo you see but that photo won't be in order then.
That's what i thought, but i wasn't sure...for some reason, i thought you could change it before on iOs devices (although i don't remember what version of iOS it was.)