Kali Linux : Expired signature key
If you cannot update your kali linux after installing in a Virtual Box or as a second boot OS, it means the certificate is expired for one of the repositories and to fix it.
Type this command below
leafpad /etc/apt/sources.list
In the sources.list file add these lines
deb http://kali.mirror.garr.it/mirrors/kali kali-rolling main non-free contrib
deb-src http://kali.mirror.garr.it/mirrors/kali kali-rolling main non-free contrib
Open terminal and type
wget -q -O - https://archive.kali.org/archive-key.asc | apt-key add