Row.Alignment (Word)

Returns or sets a WdRowAlignment constant that represents the alignment for the specified rows. Possible return values are wdAlignRowCenter - Centered, wdAlignRowLeft - Left-aligned. Default, wdAlignRowRight - Right-aligned.

Sub CenterRows() 
 ActiveDocument.Tables(1).Rows(1) _ 
 .Alignment = wdAlignRowCenter 
End Sub