Q: can we upgrade the RAM

Yes.  For example look at OWC's offerings for iMacs.  Click on your model to determine various memory upgrades avaiable for that model.  You don't have to order from them but I recommend them.

Page with instructions:

http://support.apple.com/kb/HT1423#1

Your welcome.

I should add that in addition to the apple instructions mentioned above are OWC has instructional videos showing how to change ram (and other stuff) on various macs.

Why did they remove the old RAM first in the video, is it ok to mix new RAM we buy (that meet the specs) with what's Apple installed in the factory?

Thanks.

Sometimes ram needs to be matched pairs.  Sometimes they don't play well with others.  I have no idea what video you saw.  No one in this thread ever specified which particular imacs are being talked about so I can only answer in generalities.  The specs (and any restrictions) for your particular imac are usually listed by OWC on the ram page for that imac.

Sorry, the video I was referring to was the Instructional video you posted, the mid 2011 installation video, where the instructor removed all RAM first, but I could not make out if he put the same RAM back with the new ones, or if it was of any importance, probably the best way to install is to remove existing ones first.

I read a random web forum elsewhere where someone said its best not to mix, but I'm coming to the conclusion that as long as the RAM specs are identical to Apple's spec, it should just work. Apple didn't list compatibility with multiple types of RAM, if they did then mixing would be bad, that's just my guess though.

For a mid 2011 imac the owc page for its memory just says install in pairs for best performance.  So in this case you could have two pairs of different sizes.

The video was only showing how to physically change the ram.  It's immaterial how many they were replacing, i.e., 1 pair or 2 pairs, for the purpose of that video.

That clears up a lot, looking forward to getting more RAM, totally excited at the thought!