I run ISPConfig on Debian 8 and wan't to upgrade to Debian 9. Do I have to look for something specific while upgrading? Or is there any HowTo for this upgrade?
Upgrade the OS, then run through the Debian 9 perfect server guide and rub the command to install packages to make sure nothing is missing, then update ISPConfig (download the tar file and run update.php) and let it reconfigure services.