It's relatively easy to upgrade.
Disconnect all cables from the iMac, most importantly the power plug. Simply attach a large rubber sucker to the top left hand corner of the glass, gently pull out. As the gap opens slide your hand into it and swipe to the right to fully release the top, then lift up to remove it. Remove the 8 torx screws holding the LCD in and very careful pull away from the top only enough to get your right hand in. There are now 4 cables to disconnect. First one from the backlight board in the top left corner. Next needs more care, the LCD data ribbon cable. It is short so there is little movement. Yank it out or let the LCD fall forwards and you'll damage the logic board, expensive to replace. There is a lock bar that clamps under the connector, release this by gripping the black plastic strip attached to the lever and pull up. Do not stress the socket. Ideally wear an anti static wrist band as your hand will likely brush against the logic board. Next disconnect the LCD sensor plug to the left of the plug you just disconnected. Then disconnect the ribbon cable from the bottom of the backlight board. Lift out the LCD and store safely. At this point it may be ideal to clean any dust out of the 3 fans. You will now see the 3.5" hard drive centrally at the top. Before removing the hard drive check if the airport/bluetooth board obstructs access. If it does, undo its fixing screw and swing to one side. Remove the 2 torx screws holding in the hard drive, swing out, disconnect the SATA power and data cables, remove drive. Unscrew the 2 pins and the metal bracket. Attach to a 2.5 to 3.5" converter (Crucial sell the Icy Box converter, it is ideal). Fit your SSD in or too the converter.
Now you need to decide how to deal with the iMac's sensing of the hard drives temperature sensor, if this isn't dealt with all 3 fans will slowly ramp up to a very noisy full speed. I prefer to add a temperature sensor made from a 2N3704 transistor (as Apple used on earlier models), cutting the 2 additional cables Apple add to the power connector, soldering these to the linked transistor and insulating the solder joints neatly with heat shrink tubing. The other option is to use fan control software, but this may or may not leave the processor and optical drives fan speeds controlled by their sensors. I always use the hardware fix, it only takes 5 minutes to implement. I believe you can also buy hard drive temp sensor boards for about £40.
Then reassemble and clean off any dust that may have collected on the LCD and glass, sometimes easier said than done.
Crucial could have supplied the correct RAM.
