how to upgrade RAM in iMac 2013 models

If you get a 21'5-inch iMac, I recommend you to get your Mac with the RAM you need. It's difficult to add RAM and you will void the warranty if you do that.

If you get a 27-inch iMac, you can add RAM without voiding the warranty, and Apple provides the steps > http://support.apple.com/kb/HT5540?viewlocale=en_US&locale=en_US You can install 32 GB of memory.

You can add RAM yourself if you want to without any problem in the case of the 27-inch iMac

The RAM is not user upgradeable except in the 27" model. Better decide on what you want before you purchase.

Even if you do get the 27", RAM installation is a PTA to install. Prior to setting up your computer on your desk, you must lay it it face down on a soft area (couch). Then you have to maneuver your hand & fingers either around or under the pedal to pop open the RAM door. When putting the RAM into the slots you need to use all of your God given muscles to get them in there. Otherwise, while you are seating one RAM, the one next to it will pop right on out.

When you hear the "snap," the RAM is seated properly and it's neighbor(s) will not pop out.