Here is a way to quickly update a Tails ISO image

Such a 'cold' (not live) upgrade also protects against some types of (security breaches).

Pre-requisites

You must also have or obtain right now:

• an executable xdelta3
• an executable GPG (GnuPG)
• the official OpenPGP public signing key stored in your GPG keyring, in order to check the ISO new image (it MUST BE identical to the official one published by the Tails project). To obtain this public key: download it then invoke gpg --import tails-signing.key.
  If you don't know anything about all this and don't want to explore then the present document is not for you, therefore upgrade Tails using the standard procedure. If you don't know and want to explore you may first install Tails from Debian or Ubuntu using the command line and GnuPG (then be prepared to benefit from this document).

Process (bird's view)

You will:

1. download a VCDIFF patchfile, created by me (using the 'xdelta3' software), which describes the differences between the old version of the Tails image (you already have) and the new one.
   For example a file named 'tails-amd64-6.15.1.iso_to_6.16.xdelta3' enables you to patch the Tails AMD 6.15.1 ISO image into a 6.16 version. It is useless if you don't already have the file named 'tails-amd64-6.15.1.iso'.
2. invoke 'xdelta' which will apply the patch, creating the new ISO file
3. check this new ISO image
4. use it to boot Tails

Usage

1. stop Tails (because during this process you will modify the ISO image it uses!)
2. on a command line (the directory containing your Tails ISO image file must be the current directory), invoke the 6 following commands:
   1. export Tails_PREVIOUS_VERSION=6.15.1 # replace '6.15.1' by the version number of the image file you already have
   2. export Tails_VERSION=6.16 # replace '6.16' by the version number of the newest version
   3. wget --no-clobber http://makarevitch.com/Tails/upd_xdelta/tails-amd64-${Tails_PREVIOUS_VERSION}.iso_to_${Tails_VERSION}.xdelta3 # download the pertinent patchfile. If the file doesn't exist then there is no available patch for you, sorry.
   4. wget --no-clobber https://tails.net/torrents/files/tails-amd64-${Tails_VERSION}.iso.sig # download the official GPG seal
   5. xdelta3 -d -stails-amd64-${Tails_PREVIOUS_VERSION}.iso tails-amd64-${Tails_PREVIOUS_VERSION}.iso_to_${Tails_VERSION}.xdelta3 tails-amd64-${Tails_VERSION}.iso # apply the patchfile
   6. gpg --verify tails-amd64-${Tails_VERSION}.iso.sig # check the new image file
   If it does NOT produce a gpg: Good signature from "Tails developers (offline long-term identity key)" message then something failed (it never happened to me nor to numerous friends) and DO NOT USE the new image.