Sub simple()
activecell.value=activecell.value * -1
End Sub

Tony wrote:

For something that seems so simple, I cannot come up with
a solution.

I want to run a macro that will change the contents of a
cell from a positive number to a negative number.

For example, if I have 1000 in A1, I will select A1 and
run a macro that will change its value to -1000.

Thanks in advance.