Further to Allocator's reply, you can edit the MyLog (historical) database in the RadarBox screen, but only the top part of the window - the aircraft details. You can't edit the lower part of the window - the flight details, including route details. You would need an external editor like MySqllite or whetever it's called, to edit that.
As Allocator implied, probably the important thing is to edit NavData in the way I described so that flight info is correct for the future.
Rod