How to change the sign on selected values.

I have repeated grids all linked to the primary grid. Conditional highlighting colours the fill black for positive values and white for negative. I change the pattern of black and white by changing negative numbers to positive (and vice versa) in the primary grid. Right now I do so by clicking on each cell I want to change and erasing or adding the "-". But zooming in and out, clicking 2 or 3 times to get the cursor in place, then typing is tedious and I would prefer to select a bunch of cells (not in a row or column - in irregular groups) and multiply them by -1 to change the sign. If I could even do it to one cell at a time with a click, it would be easier (because I also wouldn't have to zoom in). Any ideas on how can I do this?