How do I update my adobe Photoshop on the new iMac 24"

I'll assume you have Photoshop successfully running on your machine. All the Adobe products I've used have a "Check For Updates" utility. That may work assuming you're not having licensing problems of some sort, 32 bit vs 64 bit issue or some other "blocking" issue. Be advised, you may have already upgraded all you can on that particular app. In that case, KiltedTim is correct ... buy the new version.

You purchase the newest version from Adobe.