? this is just what is needed, follow again the steps:
https://pve.proxmox.com/wiki/Installation#Virtual_Machines_.28KVM.29
1) be sure to have a storage (nfs share or local) enabled for ISO files
2) upload your installation .iso file on that storage (select that storage, then use web gui button to upload)
3) stop the vm to be reinstalled.
4) edit its CD/DVD, and set it to use an .iso image file
5) select the storage, and then the .iso you uploaded in step 2)
6) edit the vm boot order ("options" tab, iirc)
7) in the first list, be sure to have CD/DVD, leave your vm disk in the second list.
8) start the vm, open the console (web gui button "console")
9) you should see the .iso media working, eg: a splash screen and some instruction on how to install.
10) once ended the OS install, as usual "remove" the CD/DVD, ie: simply edit your vm config and set it to use "no disk"
if you want you can skip steps 6 & 7, if you fully load the console before starting the vm (start the vm from the fully loaded console, pressing the "start" button), you should see the BIOS POST screens loading, which also inform you that you can press F12 to select the boot media, and thus select explicitly CD/DVD (to load your configured .iso), just be ready to press F12 & then select the right number proposed, or retry.
Marco