I am using Tails 2.9.1, and edit the
torrc file at
etc/tor/torrc. I have enabled persistence and edit as administrator. Edits to that file function for that session only, and are gone the next time I boot Tails.
You cannot, the persistent storage does not cater to this kind of editing, for good reason.
The ability to edit persistence is something that can be done from the
amnesia user. This user is intentionally unprivileged by default. If the
amnesia user was able to make persistent changes to
torrc this would allow an attacker who managed to exploit a process running in the context of the
amnesia user to permenantly alter the state of the system and allow them to bypass the Tails' packetfilter rules, allowing them to make arbitrary outbound connections, effectively poking a hole in it's protections.
Any changes to the filesystem (outside of the encrypted persistent storage) are only stored in ephemeral memory and are thus discarded on shutdown. The root filesystem itself it's stored in a squashfs file and is extracted on boot. While it's likely technically possible to open and manipulate the squashfs to make changes, it's possible that in doing so you'd accidentally break some more of Tails' protection (more so than you already are by editing the torrc and reducing your anonymity set).