So I've this question if it is possible to edit a standard app on SAP.
I'm currently building an app based on this one.
And I wonder if after I downloaded and uploaded to Eclipse Mars I can edit it and upload it (again) to Fiori.
Just to answer your question - Yes, you can edit it. In non S/4 systems you need a repair key to update the standard objects.
Warning: You should never do it unless in an exceptional case. Rather you should extend the Fiori app as Henning pointed.