I need to take out upstairs drain pipe and need to know how to do this .How do you cut cast iron and take it out safely ?

Cast Iron can be cut with a Sawsall using a metal cutting blade, also they make a chain type cutter that can be rented, or as I have done many times in the past, break it with a hammer, straighten up the break as good as possible with an adjustable wrench and use a Furnco rubber hub to connect to new PVC.

I need to take out upstairs drain pipe and need to know how to do this .How do you cut cast iron and take it out safely ?

Use one of these, start at the top and take it out in manageable pieces.

If you use a saws all bring plenty of blades. Cast iron's brittle and very hard.
Growler has a much better way. For a vertical and overhead cuts rent a set of ratchet cutters. While ratchet cutters will work on a horizontal pipe a set of snap cutters is faster and less of a hassle to operate. Growler also gave you good advice when he said, "start at the top and take it out in manageable pieces." Good luck, Tom