you can reboot your nodes via the GUI  or by running the reboot command on a shell or SSH session.
if you need to keep your VMs running during the reboot, you can live migrate them. for example:
1. migrate running VMs from host1 to host2
2. reboot host1
3. migrate running VMs from host2 to host1
4. reboot host2
5. migrate VMs back to host2
(containers cannot be live-migrated so you'd have to do a shutdown on those first)