The Batch Change window can be shown by selecting more than one entry or journal and then selecting "Batch Change" from the Entry menu. This shows a window that will help you make the same change across all of those selected entries and journals.
First, you need to add the property that you want to change. Use the "+" button and select what you want to change, such as "Date". That will add a line for that property to the window. Now you can set whatever value that you want. Upon clicking the "OK" button, MacJournal will apply that value to every journal and entry included. You can add multiple properties to change multiple parts of each item. If you select both entries and journals, only the properties common to both kinds of items will be available (such as their date).