There is no such command, you cannot alter a line after it's been displayed in a channel.
Now, it is possible to clear the buffer of the channel and reload the line you want, so you can display back say from line 1 to the line you want to be edited, then display the edited line and the rest of the buffer, the problem is that with big buffer, this is too slow.

Looking for a good help channel about mIRC? Check #mircscripting @