To change the sign of numbers:
Select the cell(s) that you want to change.
Go to the Data menu, and click Make Nos. Negative. Or, if you are in Easy Mode, click the Make Number(s) Negative button (in Advanced Mode, click the icon with a red minus sign).
The signs of the numbers you selected will be changed. Positive numbers will be converted to negative, and negative numbers will be converted to positive.