how to fix expired key when apt update
# run the following command:
sudo apt-key list | grep -A 1 expired
# ~< KEY >~ copy the value after the `/`
#pub 4096R/91FA4AD5 2016-12-14 [expired: 2018-12-14]
#uid MongoDB 3.6 Release Signing Key <[email protected]>
# change the <key> to what you see: ~< KEY >~
sudo apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys 91FA4AD5