When editing existing pages it is a good idea to make a backup so if things get messy you don't have to redo the whole page. This can be done by using the "Save As" button to the right of the "Save" button. Give the filename a prefix or suffix so you know what file it is the backup of. (ie- defaultBackup.htm, default2.htm, etc... ) Be sure to check the gray bar at the top of the editor for which file you are editing. If you end up editing the backup, no one will see your work because your website isn't pointing to the backup page. If this happens, do another Save As and rename it the original filename.
As you edit, save often and check your work with the preview button (If you don't see your changes, wait about 10 seconds and refresh the preview page. It takes a few seconds for the file to save on the server.).