Is it possible to change a Shimano FC131 crankset's chainrings?
No, you can't. 

The lowest Shimano level crankset with replacable chainrings is in the Acera group.

In any case, most replacement chainrings cost about as much as a Tourney (or even Acera) crankset, so it doesn't make much sense to replace the chainrings on those anyway.  

As mentioned in the answer to your other question, theres a very good chance your bike won't be able to clear a 48t ring if it was originally spec'd with a 42t ring.