For those like me who seem incapable of learning about Association
and what it can do for me, here is a brute-force method of rearranging your data:
data = Import["sales.xls", {"Sheets", "OK"}];
(* Remove headers *)
data = data[[3 ;;]];
(* Attach year and split into separate rows *)
data = Flatten[Table[{Flatten[{2014, data[[i, 1 ;; 6]]}],
Flatten[{2015, data[[i, 1]], data[[i, 7 ;; 11]]}],
Flatten[{2016, data[[i, 1]], data[[i, 12 ;; 16]]}]},
{i, Length[data]}], 1]
{{2014, "January", 1., 13., 25., 37., 49.},
{2015, "January", 61., 73., 85., 97., 109.},
{2016, "January", 121., 133., 145., 157., 169.},
{2014, "February", 2., 14., 26., 38., 50.},
{2015, "February", 62., 74., 86., 98., 110.},
{2016, "February", 122., 134., 146., 158., 170.},
{2014, "March", 3., 15., 27., 39., 51.},
{2015, "March", 63., 75., 87., 99., 111.},
.
.
.
{2016, "December", 132., 144., 156., 168., 180.}}