Major Version Upgrade on Debian

make a snapshot/backup of the machine
check the running clusters using 
pg_lsclusters command
drop the new cluster (the one running the target version) 
sudo -u postgres pg_dropcluster --stop 15 main
(optional) if you are ssh-ing from a macOS/Item2 the LC_CTYPE is wrongly set
run 
export LC_CTYPE=en_US.UTF-8 export LC_ALL=en_US.UTF-8
upgrade the old cluster 
sudo -u postgres pg_upgradecluster 13 main
check the running clusters using 
pg_lsclusters command
drop the old cluster (be sure the upgrade was successful) 
sudo -u postgres pg_dropcluster 13 main
uninstall old postgresql packages