A link to explain the IOS Upgrade Image.
I would read the page linked above as there are minimum requirements to change your OS to IOS.
Below is a link outlining the method to upgrade the CISCO AIRONET MODEL AIR-AP-1220B-x-K9 Access Point from VXWorks to IOS after downloading the IOS Upgrade Image.
Here is the URL for the root page which leads to it:
Cisco Aironet Conversion Tool for Cisco IOS Software, 1.0 Administrator Guide for Windows
NOTE: The upgrade is PERMANENT and cannot be undone. Once the AP is running IOS, there is no way of reverting back to VXWorks as the bootloader is overwritten
After loading the upgrade image, you can then download ANY IOS (not only the aforementioned upgrade image) to the AP.