Select the column header you want to move (click the letter at the top).
Cut the column:
Right-click the selected column header and choose Cut, or
Press Ctrl + X.
Select the column where you want to place the moved column.
Insert the cut column:
Right-click the destination column header and choose Insert Cut Cells, or
Press Ctrl + V to paste (and replace/move as appropriate).
If you want to drag instead:
Select the column header.
Move your cursor to the border of the selected column until it changes to a move cursor.
Click and drag to the new location, then release.
If drag-and-drop doesn’t move it:
Use Cut and Insert Cut Cells instead.