If you cannot select Different First Page, it is probably because an object is within the currently selected page or footer title. Click inside the page title or footer to make sure you're not selecting any object.
Word allows users to re-number pages from any page of the document by creating a Section Break and numbering pages from a specific number.
Step 1: Place the mouse pointer at the top of the page you want to number the page starting from. If it is the text, place the mouse pointer at the beginning of the text.
Step 2: Select the Layout tab , Next Page in the Section Breaks section from the drop-down menu appears later.
Step 3: Now the text has been created with a new section.
Step 4: Double-click on the page title or footer containing the page number you want to change.
Step 5: Click Page Number and select Format Page Numbers in the list that appears.
Step 6: In the dialog that appears, select Start at. This is the first number that you will type in the order for the page, the default is selected as 1. If desired, you can change, for example, typing the order from number 5. Then click OK.
Step 7: The page number will be played back.
Related articles:
Complete guide for Word 2016 (Part 14): Page title (Header) and footer (Footer)
Word 2016 Complete Guide (Part 12): Split text