Let's assume your positive numbers are in column A.
1) Make a new column next to it (just insert it; you're gonna delete it when you're done).
2) In column B (the one next to A), let's say you're starting with cel B2 in order to negate what's in cel A2. In cel B2, type the following formula: =-(A2)
3) Now hover your cursor over cel B2 into the lower right corner until the cursor becomes a black cross. Select with the left mouse button and pull down the length of the column. You'll have your negative numbers but you're not done yet.
4) Now select everything in column B that you want to copy over to column A. E. g. not the header cel.
5) Select and click ctrl-C
on your keyboard (that's to copy). Now go to column A (in our example, your data starts at cel A2). Right-click your mouse and select Paste Special
. Under the paste area, select Values
. Don't select or deselect anything else. Click OK
6) You can now delete column B.
Make sure not to forget step #5 as otherwise you'll lose your data, both positive and negative.
Oh yeah, step #7 - bask in your awesomeness.