# 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  

#                                 change the  to what you see:   ~< KEY >~
sudo apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys 91FA4AD5