Howtoforge shows you how to update an ISPConfig server from Debian 10 to Debian 11.
Update the ISPConfig Perfect Server from Debian 10 to Debian 11
This tutorial will take you through updating a server managed by ISPConfig from Debian 10 (buster) to Debian 11 (bullseye).
Update the ISPConfig Perfect Server from Debian 10 to Debian 11