There are 5 ways to correct work:
1. Ctrl/Cmd + Z is the shortcut command to undo the last step. If you need to get back to a step several steps ago, use the history palette or Ctrl/Cmd + Alt/Option + Z.
2. The history palette will record as many as 20 steps. Click on the step you want to go back to and observe what changes on the canvas. Activate any step by clicking on it.
3. The eraser will use the background color as it erases, giving the illusion it is subtracting pixels, when it is really just covering them up. If you are erasing on a layer with transparency, it will erase transparent. If you are working on the Background layer, it will erase to whatever the background color of the document is set to be.
By changing the opacity of the eraser, you can erase over edges of an object to make them softer looking or a paint stroke to make it softer looking. Eraser can be set to different modes, Paintbrush , pencil, block or airbrush.
4. Use the selection tools to select and delete (press the delete key) pixels from the image.
5. By using the Step Backward command over and over, you can go back step by step through the history palette . Rather than starting over again to get the effect you want, you should be able to get back to the point where you want to change an effect as long as it is not more than 20 steps back.