This post describes the upgrade process to Debian 10 aka Buster which isn’t released yet. Official release date is not announced for now but is to be expected in a second or third quarter of 2019.
For everyone who would like to profit from PHP 7.3 and other new and mostly stable software now, can perform the upgrade already now.
Before you go on, make sure you have actual full backup of the system, which gets upgraded.
Update the Debian stretch-installation to the actual patch level
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apt-get update apt-get upgrade apt-get dist-upgrade
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sed -i 's/stretch/buster/g' /etc/apt/sources.list apt-get update apt-get dist-upgrade