Build - Subnet Details
When you select a Subnet on your template, you will see a panel with details of this service on right side. You will be able to modify all options here.
Choose Existing Subnets - If you had already configured your subnets on VPC Configuration page, before start creating your template, you can choose it in this option.
Name: Subnet's name (It depends on if you are using Naming Convention)
CIDR Block: Kumolus system sets /19 by default, but you can modify it if you are creating a new subnet.