the 1.4 has single point injection, meaning only one injector. i wouldn't replce the injector with a new one, they roughtly cost £130+ each at fiat. if you are sure the injector is faulty you can replace it with a scrapyard one cheaply and easily (£5).
to remove the injector on the 1.4 you simply remove the air intake from the top of the injection unit, which looks just like a carb (because it is one, but with an injector stuck in the middle) and then reomve the injector from the centre of it. get a haynes manual and it'll be a 10 minute job.