[DNS] Update API instructions for default ACNS or TCNS (#18099)

* Replace deprecated API with instructions for new option

* Fix indentation within codeblock

Author: RebeccaTamachiro

src/content/docs/dns/nameservers/custom-nameservers/account-custom-nameservers.mdx

@@ -66,7 +66,15 @@ Cloudflare will assign an IPv4 and an IPv6 address to each ACNS name.
 
 ### 3. (Optional) Make ACNS default for new zones
 
-To make these ACNS the default nameservers for all new zones added to your account from now on, use the [Update Account endpoint](/api/operations/accounts-update-account) and set the value of `default_nameservers` to `custom.account`.
+To make these ACNS the default nameservers for all new zones added to your account from now on, use the endpoint [Update DNS Settings for an Account](/api/operations/dns-settings-for-an-account-update-dns-settings). Within the `zone_defaults` object, set the following:
+
+```txt
+"zone_defaults": {
+  "nameservers": {
+    "type": "custom.account"
+  }
+}
+```
 
 ## Disable account custom nameservers
 

src/content/docs/dns/nameservers/custom-nameservers/tenant-custom-nameservers.mdx

@@ -55,8 +55,15 @@ If the parameter `ns_set` is omitted, the default set `1` will be assigned.
 
 2. If you are **not** using [Cloudflare Registrar](/registrar/), update the nameservers at your registrar to use the TCNS names. If you are using [Cloudflare Registrar](/registrar/), no further action is needed.
 
-To make these TCNS the default namerservers for all new zones added to your account from now on, use the [Update Account endpoint](/api/operations/accounts-update-account) and set the value of `default_nameservers` to `custom.tenant`.
-
+To make these TCNS the default namerservers for all new zones added to your account from now on, use the endpoint [Update DNS Settings for an Account](/api/operations/dns-settings-for-an-account-update-dns-settings). Within the `zone_defaults` object, set the following:
+
+```txt
+"zone_defaults": {
+  "nameservers": {
+    "type": "custom.tenant"
+  }
+}
+```
 ### Disable tenant custom nameservers on a zone
 
 To remove TCNS and their associated DNS records from a zone, use a [PUT command](/api/operations/account-level-custom-nameservers-usage-for-a-zone-set-account-custom-nameserver-related-zone-metadata).