Converting all positive values in a column to negative values.

Assuming Data is column name

In a Custom column, you can put following formula

=if Number.Sign([Data])=1 then -1*[Data] else [Data]

if you don't want to use Custom column but want to do it in-place column, then next step is

=Table.TransformColumns(#"Changed Type",{{"Data", each if Number.Sign(_)=1 then -1*_ else _}})

Here you will need to replace #"Changed Type" with your previous step

If your column has all positive values only, 

= Table.TransformColumns(#"Changed Type", {{"Data", each _ * -1}})