On a test OMV 4.x system with the flashmemory plugin installed, this is all I did to upgrade it to OMV 5.x (as root):
apt-get purge openmediavault-cups openmediavault-dnsmasq openmediavault-docker-gui openmediavault-domoticz openmediavault-duplicati openmediavault-eyefi openmediavault-ldap openmediavault-letsencrypt openmediavault-mysql openmediavault-nginx openmediavault-openvpn openmediavault-pxe openmediavault-remotedesktop openmediavault-route openmediavault-shellinabox openmediavault-syncthing openmediavault transmissionbt openmediavault-urbackup-server openmediavault-vdo openmediavault-virtualbox openmediavault-webdav
sed -i "s/stretch/buster/g" /etc/apt/sources.list
sed -i "s/stretch/buster/g" /etc/apt/sources.list.d/*
sed -i "s/arrakis/usul/g" /etc/apt/sources.list.d/*
sed -i "/[Dd]ocker/d" /etc/apt/sources.list.d/omv-extras-org.list
apt-get update
apt-get dist-upgrade
reboot
apt-get purge openmediavault-omvextrasorg resolvconf
wget -O - https://github.com/OpenMediaVault-Plugin-Developers/packages/raw/master/install | bash
apt-get update
apt-get dist-upgrade
omv-salt deploy run nginx
omv-salt deploy run phpfpm
Clear your browser cache after upgrading!