Hi,
To downgrade the app service plan for your development web apps you first need to create a new App Service Plan in same region and resource group and operating system as the current App Service Plan, only with less expensive SKU such as D1. You may use link below to start the process of creating new app service plan:
https://portal.azure.com/#create/Microsoft.AppServicePlanCreate
Please remember it needs to be in same region, same resource group, and same operating system version as your existing plan.
Once the new App Service Plan is created, navigate to a development web app in the portal, on left click on App Service Plan -- Change App Service Plan blade. On right select the new App Service Plan you created above from the dropdown, and then save the change.
If you have additional development web apps that you would like to downgrade, repeat the last paragraph in order to change them to the new app service plan as well.
Please click Accept Answer and upvote if the above was helpful.
Thanks.
-TP