One of the greatest thing in Photoshop is the ability to Undo and Redo and multiple Undo. Start a new document. Pick up the Brush tool 
in the Tools Bar . We will learn about this tool in the next lessons, but from now, just select it go on the document screen click and drag to make a mark. I did this in the Photoshop.
Now you don’t like this what you made, you change your mind, To undo the step, go to Edit > Undo Brush Tool shown below.
So now it disappears as it never happened.
Now if you want to Create it again then again go to Edit Menu and now you will see ‘Redo Brush tool’ instead of ‘Undo brush Tool’. shown below.
Clicking on ‘Redo Brush Tool’ will bring back art you did with Brush. You can also use the shortcut key to undo and redo. Its Ctrl+Z .
The Step backward in the Edit Menu is also for undoing. But it relates to the history palette. So open up your History palette if its not already open from the Windows > History. Its looks like this.
Clicking on New in history palette will show you black back its the step backward. and then clicking on brush tool will again show your mark, its step forward.
Now draw some more shapes on the document with the brush tool. I did this.
Now after you have created many marks, look at your History palette, you will get each step of yours you did in Document. My Palette looks like this.
 
This shows your each step you did in Document.
If you make any mistake in the Photoshop, feel comfortable to know that you can get back to where you were, by using either the history palette or using the Undo button.
You can always go to Edit > Step backward to go to the previous step. and to step forward its Edit > Step Forward. or use the shortcuts, for Step backward its its Alt+Ctrl+Z, for Step forward its Shift+Ctrl+Z.
Now you can set how many undo you are allowed to do in Photoshop. for it go to Edit Menu > Preferences > General. or use the shortcut key Ctrl+K. Path is shown below.
It will open up a popup window shown below.
Now currently you see 20 after the History States, shown in above picture. You can choose any amount you want. Lets say you choose 50. Then its going to store 50 steps in your History palette. The Amount depends on your Memory size. If should have memory if you want more Undo. I usually keep it to Photoshop default. Because I usually don’t make mistakes 20 steps ago.
