Depending on what exactly you want there are several things you can do.
If you want to have it on a differen cell you can use a formula in a
separate column, like:
=-A1
If you want to replace on the cell, you can enter "-1" in a separate cell,
copy that cell, select the cell(s) you want to change, and go to Edit ->
Paste Especial -> Multiply.
You can even make possitive numbers to look as negative, while still being
possitive, applying a custom format like -#;# I wouldn't recommend doing
this.
Hope this helps,
Miguel.