ARM Template update existing resources

Hi, 

Im deploying WebApps using ARM and want to present these endpoints through AppGateway. App gateway is already deployed and hosting multiple backend pools/rules etc. 

Looking through the documentation i can see that you can use the ARM tempalte update resource function, but it states that you need specifiy all the existing configuration otherwise it will get over written by the update. 

So this would be a issue becuase the update is meant to be dynamic? 

Am i understanding this correctly  or is there a better way?

https://docs.microsoft.com/en-us/azure/architecture/building-blocks/extending-templates/update-resource