Hi,
Before updating agents, verify your controller version. In AppDynamics the controller should have the major version, every agent should have an equual/lower version. The version is visible in the lower side of the logon screen:
or with a browser/curl you could read the server status and your current version:
curl http://controller-ip:8090/controller/rest/serverstatus<?xml version="1.0" encoding="UTF-8"?><serverstatus vendorid="" version="1"><available>true</available><serverid></serverid><serverinfo><vendorname>AppDynamics</vendorname><productname>AppDynamics Application Performance Management</productname><serverversion>004-002-005-001</serverversion><implementationVersion>Controller v4.2.5.1 Build 37-4.2.5.next-build Commit 03b1074a2cf7999047b67187eca7fbb54d1068bb</implementationVersion></serverinfo><startupTimeInSeconds>61</startupTimeInSeconds>
If you're behind the latest version, my recommendation is to upgrade the controller. Go to http://download.appdynamics.com/ and get the latest versions (4.2.5.2) . Controller upgrade is easy, provided you have a backup, see https://docs.appdynamics.com/display/PRO42/Upgrade+the+Controller.
After upgrading, upgrade the agents downloading the new version, copy configuration in controller-info.xml and restart them. Keep in mind the agents from version 4.1 & up require you provide account-name and account-access-key in controller-info.xml or in the command line.
Hope it helps, regards,
Luis