Arcmap grid index feature

I have used the Grid index feature tool to create an index grid. The label/attribution of the page names are populated in a “row” orientated manner. eg lef to right a1,a2,a3,a4,a5,a6
b1,b2,b3,b4,b5,b6
c1,c2,c3,c4,c5,c6

How can I calculate a field so I can have the page names label/attribution in a “column” orientation?eg:
a1,b1,c1,d1,e1
a2,b2,c2,d2,e2
a3,b3,c3,d3,e3
a4,b4,c4,d4,e4