Thanks to the GPG seal published by the Tails project, you will be absolutely sure that the image file thus created is identical to the image published by the Tails project.
gpg --import https://tails.net/tails-signing.key
For example a file named 'tails-amd64-6.14.1.iso_to_6.14.2.xdelta3' enables you to patch the Tails AMD 6.14.1 ISO image into a 6.14.2 version. It is useless if you don't already have the file named 'tails-amd64-6.14.1.iso'
export Tails_PREVIOUS_VERSION=6.14.1
# replace '6.14.1' by the version number of the image file you already have
export Tails_VERSION=6.14.2
# replace '6.14.2' by the version number of the newest version
wget https://tails.net/torrents/files/tails-amd64-${Tails_VERSION}.iso.sig
# download the official GPG seal
wget http://makarevitch.com/Tails/upd_xdelta/tails-amd64-${Tails_PREVIOUS_VERSION}.iso_to_${Tails_VERSION}.xdelta3
# download the patchfile
xdelta3 -d -stails-amd64-${Tails_PREVIOUS_VERSION}.iso tails-amd64-${Tails_PREVIOUS_VERSION}.iso_to_${Tails_VERSION}.xdelta3 tails-amd64-${Tails_VERSION}.iso # apply the patchfile
gpg --verify tails-amd64-${Tails_VERSION}.iso.sig # check the new image file