To undo the last change made, navigate to Edit > Undo or press Command+Z (Mac) or CTRL+Z (Windows). Older versions of Photoshop will only be able to undo one step by default. A record of all edits is kept within the History panel, located to the right of the Canvas. Each edit is recorded as a step in the History panel. An edit is any alteration to the document, no matter how minor. Even the smallest brush strokes are each counted as an edit.

To access the History panel, click the icon that looks like a stack of three squares with an arrow. Click the step you wish to revert back to. Any steps beneath it will be grayed out, as they have been deactivated. Any additional changes made to the document will erase all grayed out steps.

Each edit is recorded as a step in the History Panel. An edit is any alteration to the document, no matter how minor. Even the smallest brush strokes are each counted as an edit.