make sure enough disk space is available, add apt source, remove old or comment out.
Example Debian 10
etc/apt/sources.list
deb http://deb.debian.org/debian buster main
deb-src http://deb.debian.org/debian buster main
deb http://deb.debian.org/debian-security/ buster/updates main
deb-src http://deb.debian.org/debian-security/ buster/updates main
deb http://deb.debian.org/debian buster-updates main
deb-src http://deb.debian.org/debian buster-updates main
then apt update
apt clean
apt-get upgrade
apt-get dist-upgrade
apt full-upgrade
apt autoremove
reboot 