Changing the ASN of an existing Azure VPN Gateway for Azure Resource Manager deployment model can be implemented by using the Azure Resource Explorer (Preview)
In the left pane drill down through the subscription, resource group, provider “Microsoft.Network”, virtualNetworkGateways and select the virtual network gateway. In this example I’ve named the gateway: VNG-Contoso
Edit the ASN number:
The ASN is now updated for VNG-Contoso.