changing a column of positive numbers to negative

Hi!

I am a newbie to numbers, and am working in Numbers 09. I have tried the other techniques that have been posted but can't make them work. In slow, easy to understand instructions 🙂 could someone please tell me how to change a column of numbers (they are in column E) to negative numbers so that i can do my banking spreadsheets. There are 995 rows in the column....

Thanks in advance!