I'm always forgetting the exact string to enter at the CLI for updating the IOS on a wireless Cisco AP, so I'll just put it here to end my future searches:
Chimp# archive download-sw /force-reload /overwrite tftp://192.168.83.150/c1100-k9w7-tar.123-8.JEC1.tar
192.168.83.150 obviously being your tftp server, and the .tar file sitting in the root of the tftp server.
I suppose if you wanted to backup your IOS you could do something along the lines of:
Chimp# archive upload-sw tftp://192.168.83.150/someimage.tar
But I haven't tried it...