On first boot the HM should come up in AP mode. To go back to AP mode after you changed to Client mode you should just have to go to Config/Network/WiFI then click EDIT on your "Radio", in the bottom section change the Mode from Client to Access Point.
That said, I know originally the HM was kinda buggy when trying to switch back and forth between Client Mode and Access Point Mode with the Edimax adapter, but I thought that had been ironed out. (though not SURE that problem was fixed up)
I don't really use AP mode, but I recall when I did experiment with it a while back the Edimax would sometimes get "stuck" in a mode and not want to switch back and forth, but my Comfast CF-WU715N adapter worked fine switching back and forth.
If you are having issues I would first suggest that you make sure your HM software is up to date (in case there is a fix out there that you do not have installed), if up to date already perhaps try the Comfast adapter or another adapter with the RT5370 chipset. However, I do recall some people reporting other adapters that should have the RT5370 chipset were not working with the rPi (not the comfast one I listed, but others), so that may be a crap shoot as well if you just try to pick an adapter with that chipset.