Freelander TD4 injectors (replace)

I have been told by a garage when they did my Clutch master cylinder that one of the injectors isn't working properly. He said don't change them unless it starts missfiring or wont start. I have just ordered a Haynes manual thinking of changing them myself. How easy is it to remove and replace them. I have seen a company that will check and supply refurbed injectors by post at a reasonable price.???