Thanks to such a 'cold' (not live) upgrade you are protected from some types of (security breaches).
gpg --import https://tails.net/tails-signing.key
For example a file named 'tails-amd64-6.15.iso_to_6.15.1.xdelta3' enables you to patch the Tails AMD 6.15 ISO image into a 6.15.1 version. It is useless if you don't already have the file named 'tails-amd64-6.15.iso'
export Tails_PREVIOUS_VERSION=6.15
# replace '6.15' by the version number of the image file you already have
export Tails_VERSION=6.15.1
# replace '6.15.1' 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_${Tails_VERSION}.xdelta3
# download the pertinent patchfile
xdelta3 -d -stails-amd64-${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