Adobe Photoshop CS6 User Manual page 67

You can also use the History panel to delete image states and, in Photoshop, to create a document from a state or snapshot.
To display the History panel, choose Window > History, or click the History panel tab.
Photoshop History panel
A. Sets the source for the history brush B. Thumbnail of a snapshot C. History state D. History state slider
Keep the following in mind when using the History panel:
Program-wide changes, such as changes to panels, color settings, actions, and preferences, are not reflected in the History panel, because
they are not changes to a particular image.
By default, the History panel lists the previous 20 states. You can change the number of remembered states by setting a preference. Older
states are automatically deleted to free more memory for Photoshop. To keep a particular state throughout your work session, make a
snapshot of the state.
Once you close and reopen the document, all states and snapshots from the last working session are cleared from the panel.
By default, a snapshot of the initial state of the document is displayed at the top of the panel.
States are added to the bottom of the list. That is, the oldest state is at the top of the list, the most recent one at the bottom.
Each state is listed with the name of the tool or command used to change the image.
By default, when you select a state, the states below it are dimmed. This way you can easily see which changes will be discarded if you
continue working from the selected state.
By default, selecting a state and then changing the image eliminates all states that come after it.
If you select a state and then change the image, eliminating the states that came after, you can use the Undo command to undo the last
change and restore the eliminated states.
By default, deleting a state deletes that state and those that came after it. If you choose the Allow Non-Linear History option, deleting a state
deletes only that state.
Revert to a previous image state
Do any of the following:
Click the name of the state.
Choose Step Forward or Step Backward from the History panel menu or the Edit menu to move to the next or previous state.
Delete one or more image states
Do one of the following:
Click the name of the state, and choose Delete from the History panel menu to delete that change and those that came after it.
Drag the state to the Delete icon
Choose Clear History from the panel menu to delete the list of states from the History panel, without changing the image. This option doesn't
reduce the amount of memory used by Photoshop.
Hold down Alt (Windows) or Option (Mac OS), and choose Clear History from the panel menu to purge the list of states without changing the
image. If you get a message that Photoshop is low on memory, purging states is useful, because the command deletes the states from the
Undo buffer and frees up memory. You can't undo the Clear History command.
Choose Edit > Purge > Histories to purge the list of states for all open documents. You can't undo this action.
to delete that change and those that came after it.