If you flatten a document or merge layers in Adobe Photoshop CC, there are a couple of ways you can pry them apart again: use the Undo commands or the History panel. Once you get your project back to the way you need it, there are two alternatives for managing layers you should check out, which give you many of the benefits of merging, without having to unmerge layers later.
Using the Undo Commands
If you haven't made any other changes to a Photoshop project after merging layers, you can undo the merge from the Edit menu. Select Undo Merge or Undo Flatten, depending on which task you performed. Alternatively, just press Ctrl-Z, to undo the last action you performed.
If you have performed other actions on the file since you last merged the layers, you can trace back through your steps by pressing Alt-Ctrl-Z, or selecting Step Backward from the Edit menu, multiple times. Keep an eye on the Layers panel to see when the layers are returned to the non-merged state. Keep in mind that you lose all of the work that you did after the merge.
Using the History Panel
Photoshop's History panel is like a wormhole for graphic designers. Instead of tediously going back one step at a time, open the History panel from the Windows menu and then click on the state just before you merged the layers. Note that you will lose all of the work you did since the merge.
The History panel works only for actions you have completed in your current Photoshop session. If you close Photoshop and open a saved file, it won't give you access to anything that you did in the last session.
Alternatives to Merging or Flattening
Whenever you want to merge two or more layers, you can do it without having to unmerge them again later. Two good alternatives are to use folders or to create a new merged layer.
Group Instead of Merge
Grouping layers into a folder gives you many of the benefits of merging while keeping each layer intact. Ctrl-click each layer to group in the Layers panel and then click the Folder icon. This puts the layers into a single folder, which you can move up and down in the Layers panel and edit at the same time.
To work on only one layer in the folder, click the Arrow icon beside the folder and then select that layer. To remove a layer from a folder, just drag it out.
Merge Into a New Layer
Photoshop has a keyboard shortcut that merges all visible content into a new layer without affecting the layers below it.
- Click the Eye icon beside any layers you don't want merged to hide them.
- Press Ctrl-Alt-Shift-E. A new layer appears with the merged content. Mac users can press Command-Option-Shift-E.
- Click the Eye icons to make the hidden layers visible again.
- Click the Eye icons beside the layers you merged to hide them without deleting them from your file.