Edit a reference, how? In endnote it is automatically saved, when you edit a record and close it. You can do a ctrl+S to be sure, but it is not required. You can also close the record with ctrl+W and it will save it before closing the record window. Even if you edit it from the reference panel on the side/bottom (whereever you have defined in the layout tab) is automatically saves it.
The only thing you do need to do - if you change it in error, and want to undo a change, before you close it, you do need to File>revert reference (no keyboard shortcut or on toolbar).