Use the Modify > Change Parameters to open the table with the properties. Each is described (hopefully these descriptions make sense). 

The model provides a flange for the square pipe to go into, and a fitting that goes into the round pipe. 

The model should automatically update. 

For example, 63mm square gutter pipe to 120mm waste water pipe. 

Or, 65mm square gutter pipe to 70mm waste water pipe, but they are offset 10mm x and 20mm y. 

Note that in order to make a lofted pipe in fusion360, the only way I was able to make it happen is to loft the inside (make a solid), then loft the outside (as a separate body), then subtract the inside from the outside. 
That's all done in the model so you don't really need to worry about it.