Narrated Al-Bara:
When the Verse:--"Not equal are those of the believers who sit (at home)," (4.95) was revealed, the Prophet said, "Call so-and-so." That person came to him with an ink-pot and a wooden board or a shoulder scapula bone. The Prophet said (to him), "Write: 'Not equal are those believers who sit (at home) and those who strive and fight in the Cause of Allah." Ibn Um Maktum who was sitting behind the Prophet then said, "O Allah's Apostle! I am a blind man." So there was revealed in the place of that Verse, the Verse:--"Not equal are those of the believers who sit (at home) except those who are disabled (by injury, or are blind or lame etc.) and those who strive and fight in the Cause of Allah." (4.95)
Sahih Bukhari, Volume 6, Book 60, Number 118
So basically it's like this:
Muhammad: I just got a revelation from Allah. Bring me the scribe!
Scribe: Here I am!
Muhammad: Cool, now write this for me: "Bros who sit their lazy asses at home are not as awesome as those who kick ass in the name of Allah."
Ibn Um Maktum: But, sir, I am blind, thus I cannot fight! Does this mean I'll never be as awesome as others?
Muhammad: Hey, you got a point there! Okay scratch that, here's the revised version: "Bros who sit their lazy asses at home are not as awesome as those who kick ass in the name Allah, unless you have disabilities like blindness or deaf or some shit like that, which in that case you'd still be awesome."
Ibn Um Maktum: Oh prophet Muhammad, you trully are a man of understanding!
Muhammad: No probs bro.
So in this haddith, we know that Muhammad actually had a revelation of the Quran, that is supposedly
immutable, and he basically changed it himself after hearing the plight of his scribe.