1. Select VPS in billing panel.
2. mount iso image in billing panel
3. open noVNC console
5. push ESC button on keyboard when PROXMOX logo shows
At this step, you need to act quickly, since this inscription is displayed on the screen for a short time.
5. choose boot from CDROM
Usually all ISO images are net-install edition and you need use network configuration which was given to you.
For more information about network settings, see: What are the network settings in my VPS?