If you are already running 6.2 you have to install the normal firmware image to update to 6.2.1 e.g. when prompted when you open the web admin UI
The special image for going from 4.2.x to 6.2 has modified headers to make the 6.2 image appear to be a 4.2.x one. The image to go back from 6.2 to 4.2.x has modified headers that make the 4.2.x image appear to be an OS6 one.
The special images can only be used when going from 4.2.x to 6.2 or vice versa (note each of these requires a factory default, so backup your data first). If you try to use the special image to update from e.g. 6.2.0 to 6.2.1 the system will reject it as OS6 does not accept 4.2.x images.
