You don't. You can only convert units when they're both units of the
same physical quantity. In this case, 'newton' is a unit of force, but
'kilogram' is a unit of mass. Those are not the same physical quantity,
so they don't convert.
The closest you can get is to find the weight of some mass when you know
the gravity in the place where the mass is. The weight of some mass is
(mass in kilograms) x (gravity in meters per second²)
and the answer is in newtons.
On Earth, the gravity is 9.8 meters per second², so each kilogram of mass
weighs 9.8 newtons.
In other places, like on the moon or on Mars, the gravity is different, so
a kilogram weighs something different.
