If you want precise control over each grid item on the layout, right-click the grid and click Convert To Graphics 
. Once converted, the grid appears as a grouped element in the Contents pane. You now have precise control over where you place each element, but the grid has lost its dynamic properties. If you change the map frame size after you convert the grid to graphics, the grid will not update and may not line up with your resized map frame.
To make changes to one or several grid graphics, expand the grouped element and select the text, line, or lasso graphic. You can change each type of graphic in its respective Element pane.