I have heard that the Quran has been changed and corrected a few times.
Could you please shed more light on this? i.e. where? when? by whom?

Although I have read the entire Quran, as well as a number of books on
Islam, I do not feel I am an expert on this subject. Yes, apparently,
there have been a number of editions, changes, deletions etc. to the Quran
since its initial edition some time soon after the death of Muhammed. I
would refer you to Douglas Jacoby, who answers questions from the web site
www.douglasjacoby.com. In fact, I am presently writing him for a more
solid answer.

Here is a response from Douglas Jacoby:

Sure, I have some material on this. One of my best sources has been
Geisler’s Answering Islam, to which I would point your friend. There is
also a website connected with Answering Islam–why not check it out? (I
haven’t yet.)

What I would add is that it is not clear how significant such supposed
changes would have on the essential character of the Quran or the central
doctrines of Islam. If I understand correctly, such editing and possible
additions and deletions will probably not have a huge effect in the bottom
line on Muslim teaching or doctrine. One reason for this is that the Quran
is quite repetitive. Many of the sayings contained in it are repeated
either exactly the same or in very similar ways in several places
throughout the book. It is not clear that taking a couple of surahs out
would have a major effect. I am not aware of any supposed additions to the
Quran which are highly controversial or which would have a huge effect on
the religion. Besides, the Quran, even if changed in its early lifetime,
has remained unchanged since fairly early in its history. One will not
find evidence for changes several hundred years after the initial edition
of the Quran.

Sorry to give such a vague answer, and in the mean time, I am doing some
research to see if I can find a better answer.

John Oakes, Ph.D.