Added sample code for Set-AzFirewall which updates existing rules (#12045)

* Update Set-AzFirewall.md

Added an advanced example to update existing Azure Firewall rules using PowerShell based on customer feedback.

* Update ChangeLog.md

* Update ChangeLog.md

no need to update changelog for help changes

Co-authored-by: Yabo Hu <[email protected]>

Author: teilmeier

src/Network/Network/ChangeLog.md

@@ -20,7 +20,6 @@
 
 ## Upcoming Release
 * Added support for AddressPrefixType parameter to `Remove-AzExpressRouteCircuitConnectionConfig
-
 * Added name validation for `New-AzIpGroup`
 
 ## Version 3.0.0

src/Network/Network/help/Set-AzFirewall.md

@@ -122,7 +122,7 @@ $azFw | Set-AzFirewall
 
 In this example, the management public IP address of the firewall will be changed to "AzFwMgmtPublicIp2"
 
-### 6:	Add DNS configuration to an Azure Firewall
+### 9:	Add DNS configuration to an Azure Firewall
 ```
 $dnsServers = @("10.10.10.1", "20.20.20.2")
 $azFw = Get-AzFirewall -Name "AzureFirewall" -ResourceGroupName "rg"
@@ -134,6 +134,17 @@ $azFw | Set-AzFirewall
 
 In this example, DNS Proxy and DNS Server configuration is attached to the Firewall.
 
+### 10:	Update destination of an existing rule within a Firewall application rule collection
+```
+$azFw = Get-AzFirewall -Name "AzureFirewall" -ResourceGroupName "rg"
+$ruleCollection = $azFw.GetNetworkRuleCollectionByName("ruleCollectionName")
+$rule=$ruleCollection.GetRuleByName("ruleName")
+$rule.DestinationAddresses="10.10.10.10"
+Set-AzFirewall -AzureFirewall $azFw
+```
+
+This example updates the destination of an existing rule within a rule collection of an Azure Firewall. This allows you to automatically update your rules when IP addresses change dynamically.
+
 ## PARAMETERS
 
 ### -AsJob