The only way to unmerge layers is to step back in your history.
This form of editing is known as ‘destructive photo editing’ since you’re making a permanent change to your layers. Yes, you can ‘undo’ the changes, but once a layer is merged, you’re committed. The problem with merging or flattening layers is that it’s completely permanent. Fortunately, there are a few simple ways you can unmerge layers in Photoshop using some easy commands to get all your layers back. Multiple layers now have become one, and there’s no way to edit them individually. Whether you merged or flattened your layers, the result is going to be the same. So you’ve run into a bit of a snag, and you can’t figure out how to unmerge layers in Photoshop.