If you are creating a column chart with both negative and positive values, and the last data series has negative numbers, the value label will show up at the bottom of the chart.
If you want all the value labels to be at the top of the series, you will need to make sure that the last data series you select has positive numbers. Or you can rearrange the order of the data series by clicking and dragging. Again if the last data series is a negative number, the value label will appear at the bottom of your chart.
This gif illustrates how moving a negative data series from the bottom of a list changes the position of the value label.