clarify PUT request body

Author: ranbel

src/content/docs/cloudflare-one/identity/service-tokens.mdx

@@ -41,7 +41,14 @@ You can configure a self-hosted Access application to accept a service token in
 
 To authenticate using a single header:
 
-1. In your Access application, specify the name of the header you want to use for service token authentication:
+1. Get your existing Access application configuration:
+
+	<APIRequest
+		path="/accounts/{account_id}/access/apps/{app_id}"
+		method="GET"
+	/>
+
+2. Make a `PUT` request with the name of the header you want to use for service token authentication. To avoid overwriting your existing configuration, the `PUT` request body should contain all fields returned by the previous `GET` request.
 
 	<APIRequest
 		path="/accounts/{account_id}/access/apps/{app_id}"
@@ -51,7 +58,6 @@ To authenticate using a single header:
 			"type": "self_hosted",
 			"read_service_tokens_from_header": "Authorization"
 		}}
-
 	/>
 
 2. Add the header to any HTTP request. For example,