You'll need to ask this on the Cisco Enterprise community.
Just a quick warning; when upgrading to newer software, especially if you are doing a big jump, check the AP compatibility matrix that your APs are still supported (unless they are relatively new models).
You should be fine if you stick within the 8.4.x train that you are currently using.
I've seen people upgrade te 2504's several times with old AP models only to find their APs don't work anymore because they are no longer supported.
To get you going in roughly the right direction, download the software from:
I recommend you use a "gold star" release if you can.
And then follow this software upgrade guide.