Once you have copies of the key, check the fingerprint:
gpg -n --import --import-options import-show unman.pub
replacing unman.pub with the path to the key.
This will show the fingerprint of the primary key:
pub rsa4096 2016-06-25 [SC]
4B1F 400D F256 51B5 3C41 41B3 8B3F 30F9 C8C0 C2EF
uid [ unknown] unman (Qubes OS signing key)
sub rsa4096 2016-06-27 [S] [expires: 2027-06-30]
sub rsa4096 2016-06-25 [E]
I use a subkey to sign packages. You can read about the idea behind this in this post by Alex Cabal:
Creating the Perfect GPG Keypair
You can see details of the subkeys by running:
gpg --show-keys --with-subkey-fingerprint unman.pub
pub rsa4096 2016-06-25 [SC]
4B1F400DF25651B53C4141B38B3F30F9C8C0C2EF
uid unman (Qubes OS signing key)
sub rsa4096 2016-06-27 [S] [expires: 2027-06-30]
7473414DF9A181A45244BDACFDD1B8244731B36C
sub rsa4096 2016-06-25 [E]
F4A29C5E2E90B2E0732358E46233CD8FA59A87A8
The signing subkey is marked with "S".
Check that it is in date.
Once you are satisfied that you have a genuine key, you can use it to validate packages, or install templates.
qvm-run -p QUBE 'cat PATH_TO_KEY' > unman.asc
sudo rpmkeys --import PATH_TO_KEY
/etc/qubes/repo-templates/keys
You can then use qvm-template as normal, either to manually install templates, or to install from the 3isec repository.
/etc/qubes/repo-templates/keys, overwriting the old key.
sudo rpmkeys --list
sudo rpmkeys --delete UUID
sudo rpmkeys --import NEW_KEY
UUID is the UUID of the unman signing key from the first command.
unman