Not exactly sure how you're attempting to do this.
with VBA, or is just something you need once.
For a column? or just a single cell?

the answer that was given as: "multiply by -1" works.
in VBA, you could just use: Cell(2,1) = -1 * cell(2,1)
If the number  is already negative  though, it will reverse  the sign.
To ensure it will always result in a negative number,
try: Cell(2,1) = -1 * abs(cell(2,1))