multiply by negative  one........

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))

You don't need the multiplication, just the negation.

Cell(2,1) = -Cell(2,1)

perhaps you don't know *where* to
put the code in order to get the code to execute in order to sign  to
change.

You can put the code inside the SelectionChange (or the Change)
event that belongs to the sheet the data is in. Then, when a number
is entered and the Enter key pushed, this Sub will fire and the sign
can be changed with the code.

In the VB editor, select the sheet, then select Worksheet in the
left Object PullDown on the top left. The followinag should appear
automatically. If it does not, select it in the upper right Procedure
PullDown.

Option Explicit
.................................................
Private Sub Worksheet_SelectionChange(ByVal Target As Excel.Range)
' Put the sign change  code here
End Sub
.............................................

=(A1) - (A1*2)..............................