If a passenger's name is spelled wrong on a flight I have already ticketed, can it be corrected?

Sometimes yes, but it depends on the airline and on how big the mistake is, so please do not assume a correction will always be possible.

Airlines draw a line between a small spelling slip and a change of passenger. A letter or two out of place in a first name, or a missing middle name, is often treated as a correction and many airlines will allow it, sometimes free and sometimes for a fee they set themselves. Swapping one traveller for another is a different matter and most airlines simply refuse, because the ticket is issued to a named person and is not transferable. Anything in between, such as a surname that is wrong in several places or a first name and surname entered the other way round, sits in the airline's own judgement.

How the request reaches the airline also matters. A correction is not something your portal can apply on its own, because the airline holds the ticket record. The request goes to the supplier behind the flight, they raise it with the airline, and the answer comes back in the airline's own time. That can take a while, and if the departure is close there may not be enough time at all, which is why catching the error early makes a real difference.

If you spot a wrong name, raise it with your flight supplier straight away rather than waiting, and check the passport spelling against the booking before you issue the ticket so the question does not come up in the first place.