I have come across some answers that are now incorrect since the launch of iOS5. Such as this one
Should another answer be put down or should we change these answers to make them factually correct?
Moderators don't generally delete things because they are incorrect or out of date. This is one of the stock flag rejection messages: "Flags should not be used to indicate technical inaccuracies, or an altogether wrong answer".
It's okay to edit, but you generally shouldn't change the meaning of the original post. This is one of the stock edit rejection messages: "This edit changes too much in the original post; the original meaning or intent of the post would be lost."
Note, of course, that if the post is a Community Wiki you should by all means change it as much as necessary.
Generally, the thing to do is write your own answer!
If your new one is correct, people will vote it up and it will float to the top.
It's also completely okay to comment on an out-of-date answer to warn the OP and other users.
I would probably stop short of downvoting until the OP has had fair warning that their post is out of date, so that they have a chance to fix it.