History palette

You can use the History palette to revert to a previous state of an image, to delete an image's states, and in Photoshop, to create a document from a state or snapshot. Photoshop History palette A. Sets the source for the history brush B. Thumbnail of a snapshot C. History state D. History state slider. To display the History palette: Choose Window > History, or click the History palette tab.

To revert to a previous state of an image:

Do any of the following:
 1) Click the name of the state.
 2) Drag the slider at the left of the state up or down to a different state.                                               
 3) (Photoshop) Choose Step Forward or Step Backward from the palette menu or the Edit menu to move to the next or previous state.

To delete one or more states of the image (Photoshop):

Do one of the following: 
 1) Click the name of the state, and choose Delete from the History palette menu to delete that change      and those that came after it.
 2) Drag the state to the Trash button   to delete that change and those that came after it.
 3) Choose Clear History from the palette menu to delete the list of states from the History palette, without changing the image. This option doesn't reduce the amount of memory used by Photoshop.
 4) Hold down Alt (Windows) or Option (Mac OS), and choose Clear History from the palette menu to purge the list of states from the History palette without changing the image. If you get a message that Photoshop is low on memory, purging states is useful, since the command deletes the states from the Undo buffer and frees up memory.

Important: This action cannot be undone.
Choose Edit > Purge > Histories to purge the list of states from the History palette for all open documents.
Important: This action cannot be undone.

To delete all of an image's states (ImageReady):

Do the following:
Choose Clear Undo/Redo History from the History palette menu.
Important: This action cannot be undone.

To create a new document from the selected state or snapshot of the image (Photoshop):

Do one of the following:
1) Drag a state or snapshot onto the New Document button.
 2) Select a state or snapshot, and click the New Document button. 
 3) Select a state or snapshot, and choose New Document from the History palette menu.

The history list for the newly created document will be empty.