If you don’t want to save changes you made to your document since opening it or last saving it, you can undo them.
To undo your most recent change, choose Edit > Undo or press Command-Z.
To undo multiple changes, choose Edit > Undo multiple times (or press Command-Z) multiple times. You can undo any changes you made since opening the document or reverting to the last saved version.
To undo one or more Edit > Undo operations, choose Edit > Redo (or press Shift-Command-Z) one or more times.
If you’re using a computer with OS X Lion (v10.7) or later, you can revert to previously saved versions of your document. To learn how, see Finding an Archived Version of a Presentation.
If you’re using a computer with Mac OS X v10.6 (Snow Leopard) or earlier, you can undo all changes you made since the last time you saved your document by choosing File > “Revert to Saved,” and then clicking Revert.
Choose File > “Revert to Saved.” The changes in your open document are undone.