You need to multiply all of the cells by -1. Find an empty cell
somewhere, out of the range that you intend to change, and put -1 into
that cell. Select the cell and then click <copy>. Then move the cursor
and highlight all the cells that you want to change. Right-click in
that area and select Paste Special, then Values (check) and Multiply
(check), then OK followed by <Esc>. Then you can delete the -1 from
the original cell.
Hope this helps.
Pete