How to Convert Newton to N-m?

Multiplying with distance

You can't convert one to the other as they are different things, one is a force, the other is a torque.
The answer is to look at the specifics of the problem.
Newton is a force, imagine 1N applied to a long spanner to tighten a nut.
If the spanner is 1m long then you are applying 1Nm of torque. If the spanner is 0.5m then you are only applying 0.5Nm of torque.
So you see it depends on the geometry of the actual application.
Del _{(I have a very big spanner)}