When you select a page from the page tree you will see all its contents elements listed. Above each one you will see a small set of icons as shown in Fig. 1. To copy a content element:

1. Click on the content element icon (1 in Fig. 1) and select copy;
2. Click on the content element icon again and select Paste after.

You will now have a second copy of your content element. Click on the hide icon (2 in Fig. 3) to hide it. 

You can now edit your hidden content element and, when it is ready unhide it and delete the old version.