a vm or a container?
a vms network is configured completely inside the guest, no possibility from proxmox directly
but you can use the novnc console on the web to go into the guest
for containers you can use the pct tool
e.g., pct enter <VMID> to enter and correct the setting
or pct set <VMID> -net0 <OPTIONS> for directly configuring the network settings
(see man pct for details)