You can use the "system-switch-displaymanager" tools to manage the display manager. This is possibly the most portable way.
To install it:
# dnf install system-switch-displaymanager system-switch-displaymanager-gnome
And choose the display manager after running
You will need to restart X server to make it take effect.
If you want to know what's happening (and it may change in the future):
# ls -l /etc/systemd/system/display-manager.service
It points to the display manager's service.