Add clearer upgrade instructions

Author: Peter Kelly

CHANGELOG.md

@@ -8,10 +8,18 @@ IMPROVEMENTS:
 
 UPGRADE:
 
-* Remove the previous version of nginx-asg-sync e.g. `dpkg --remove nginx-asg-sync`
-* Deploy the new version to your NGINX Plus instance in AWS and install it `dpkg -i nginx-asg-sync_0.2-1-xenial_amd64.deb`
-* Update the `/etc/nginx/aws.yaml` to the new format (example in the [configuration section](https://github.com/nginxinc/nginx-asg-sync#nginx-asg-sync-configuration) of the README.md))
-* Reload NGINX Plus
+The upgrade process requires changing both NGINX Plus configuration and nginx-asg-sync configuration. Below are the recommended steps to follow:
+
+1. Upgrade NGINX Plus to R14 or R15
+2. Enable the new API in the NGINX Plus configuration while keeping the upstream_conf and the status API enabled. See an example of configuring the new API in the configuration section, but make sure to keep the upstream_conf and the status API.
+3. Reload NGINX Plus to apply the updated configuration
+4. Modify the /etc/nginx/aws.yaml file:
+    * Remove the `upstream_conf_endpoint` and `status_endpoint` fields.
+    * Add the `api_endpoint` field. See an example in the configuration section of the README.md
+5. Download the Release 0.2 nginx-asg-sync package for your OS and upgrade the package using the OS tools (dpkg or rpm).
+6. Check the logs of nginx-asg-sync to make sure that it is working properly after the upgrade.
+7. Finally remove the upstream_conf and the status API from NGINX Plus configuration.
+8. Reload NGINX Plus to apply the updated configuration
 
 Note: the supported versions of NGINX Plus are R14 and higher.