PageNumber.Alignment (Word)

Returns or sets a WdPageNumberAlignment constant that represents the alignment for the page number.

Possible Values are wdAlignPageNumberCenter - Centered, wdAlignPageNumberInside - Left-aligned just inside the footer, wdAlignPageNumberLeft - Left-aligned, wdAlignPageNumberOutside - Right-aligned just outside the footer, wdAlignPageNumberRight - Right-aligned.
Selection.HeaderFooter.PageNumbers(1).Alignment = wdAlignPageNumberCenter