Save first to make sure we don't mess it up
This works if the values you want to change are all positive. It won't work for values that are already negative.
Select the cells that you want to make negative. Right click the selection and choose COPY.
In an empty column maybe next to the cells you selected if it is empty, right click a cell that you want to be the top cell in your list.
Choose PASTE SPECIAL
Under OPERATIONS click SUBTRACT
That should make a column of negative versions of what you had selected. Then you can copy those and paste them over the positive numbers.
Hope this helps