You can actually edit the official maps

[Mr Argon]

not edit at all, but you can move tiles in the editor in official map if you move it and without releasing left mouse button you press the right mouse button(the menu opens), and then click anywhere on the screen, then you can actually move the tile.

I edited the first campaign and accidentally saved it, then the next time I opened it, it showed me the "you can't edit the map while it's locked" sign, but I don't know if the change has been made at all, because I don't remember what did I change.

[JakSparro98]

This kind of problem was discussed in the closed beta stage of the game, the conclusion at that time was this:

Really you cannot, only a popup is shown to prompt if you want to save when you close the map, but even if you choose yes the map won't be touched at all.