Is there a way to revert to vehicle assembly during flight?

@Zenithspeed Thank you very much this solved my question, have a nice day/night.

There is no way to edit the craft during flight.
You must end flight and undo flight to get back to design.

what you may be looking for is the "undo launch" button, when you're in the pause menu hit the button that you would normally do to exit, but instead go to "retry/undo" (should be the middle one), and after that click "undo flight" (should be on the right) which will basically remove the craft and revert time to when launched, as well as being you to the designer or main menu (depending on if you launched it from menu or from designer obviously)

I'm not sure I know what exactly you mean? you mean go back an change the craft after you have launched it? The only way I know to do that is .... in flight "save location" you will then have to delete that craft but with the location saved you can then upload any craft you choose and in a sense replace the old one!