I noticed last week that Apple Music had managed to allocate album artwork incorrectly. So I opened iTunes on the Mac, selected the relevant affected tracks/albums & used ⌘i to reveal the existing artwork. I dragged copies of each one to the desktop then selected each album & proceeded to edit multiple items.
First I revealed the current incorrect artwork then deleted it for each album in succession. Dragging the copied artwork from the desktop to the relevant spot ion the Get Info screen replaced the artwork in each album.
I then waited a few minutes, checked the Music App on my iOS devices & was pleased to note the the iCloud Music Library had replicated the changes.