Frist, You SHOULD NOT try to name the file the exact same name, but give it a slightly different meaningful name so you know what is changed and change the document to use the new named file.
If you still can’t save it (it keep adding the word copy to the name?) Also Check your preferences. You CAN NOT have the folder locations pointing to the installed styles folder in endnote’s program folder (it is write protected and you can’t save there when the program is open, at least in Windows). You should have the styles folder pointer to a user area.
This is for two reasons.
One is it is write protected as I said above.
Two, when you update a style in the installation styles folder version would be erased and updated with the new endnote versions during updates or upgrades.