You can use Set-AzWebApp cmdlet. For a complete list of PowerShell commands that you can use for App Service, see https://learn.microsoft.com/en-us/powershell/module/az.websites/?view=azps-6.5.0#app_service.
Your command will look something like Set-AzWebApp -AppSettings <Hashtable> -Name 'ContosoWebApp' -ResourceGroupName 'Default-Web-WestUS'
where <Hastable>
is @{ <setting-name> = <value>; }
. The values inside the hashtable will update if existing or add if missing.