If you have any 3600 series AP's such as AP3610 you must stay on the 9.21 track. The 9.21.20 is the latest release of that track. If you only have 3700,3800 and 3900 series AP's you can upgrade to our latest 10.41.10,or the soon to be available 10.41.11 code but you will need to have your controller covered under a service contract as you will need to request V10 license keys as the V10 requires a new license.