Fedora Magazine - https://fedoramagazine.org/use-lvm-upgrade-fedora/
Most users find it simple to upgrade from one Fedora release to the next with the standard process. However, there are inevitably many special cases that Fedora can also handle. This article shows one way to upgrade using DNF along with Logical Volume Management (LVM) to keep a bootable backup in case of problems. This example upgrades a Fedora 26 system to Fedora 28.
The process shown here is more complex than the standard upgrade process. You should have a strong grasp of how LVM works before you use this process. Without proper skill and care, you could lose data