Current draft here: http://h.ssb.gathman.org/files/server-upgrade.html (need Cjdns installed to connect)
Excerpt: This article shows one way to upgrade a Fedora server using DNF along with Logical Volume Management (LVM) to keep a bootable backup in case of problems, and minimize downtime. This example upgrades a Fedora 33 virtual machine host to Fedora 35.
This example assumes you already have libvirt, qemu-kvm, partclone, and python3-dnf-plugin-system-upgrade installed. You must have enough memory available (2G recommended but I have succeeded with 1G) to create an additional virtual machine to run the upgrade. Your system can continue to operate while you prepare for and download the upgrade, and while the upgrade runs. Before you start, ensure your existing system is fully updated.