You're trying to write the equivalent of a mutator method: http://en.wikipedia.org/wiki/Mutator_method

The alternative to a mutator is a function, which takes in an argument and returns something. In this case, a function would take a list and return the same list but a new value as it's second part. That would be this for example f[lst_]:=ReplacePart[lst,newvalue,2]. You can change the value of a list by running lst = f.

In programming you often have the choice between a mutator method and a function. Without going into too much detail, a function is generally preferable (unless there's a good reason). In fact a lot of programming languages don't allow mutators.

You can write a mutator method in Mathematica. Here's an example of a function that increments it's input by one:

SetAttributes[f, HoldAll];f[x_] := (x = x + 1);

I've use HoldAll as an attribute to prevent x from evaluating into a number which would present Set (=) from working.

Hi,
I wonder if you meant to type ss(number) (These brackets should be square, but this editor erases them) which is what produced the "setps" error for me.  If so, the specific error happens because when ss[2] is called, it results in evaluating the first function x[[2]] = 3, resulting in 2[[2]] = 3 which is not syntactically correct. "Print" by itself will also not produce an output, it must have something fed to it as an argument. 

If you were wanting to just replace the second element of the list "a", it is easy to do as follows.