If the values are all positive and you want to make them negative then
you can multiply by -1 (more on this later). However, if you have a
mixture of positive and negative values in the column, you would need
to apply a formula in a helper cell along the lines of:
=-ABS(A1)
To multiply all cells by -1, enter -1 in a blank cell somewhere. Then
select that cell and click <copy>. Then highlight all the cells that
you want to change and click on Edit | Paste Special | Values (check)
| Multiply (check) | OK then <Esc>.
Hope this helps.
Pete