Which brand is really trusted on RAM upgrade?

Personally, I have always used OWC's RAM and it has always worked well for me, first time / every time. Kurt Lang is right (and maybe not emphasizing enough), changing / adding RAM on that Mac is not easy. As I understand it Apple will not undetake RAM upgrades after the sale of the Mac. A certified Apple reseller / repair operation probably will but it will be expensive. They have to crack open the case (remove the screen) to access the RAM.

Another vote for Crucial or OWC (www.macsales.com). Never use any other brand unless you want to risk compatibility or failures. While even Crucial or OWC can have failures the percentage is dramatically lower and compatibility is simply a non-issue.

By far, Crucial and OWC are the ones most users here would go with first. They both have 100% forever guarantees. No matter how old the RAM you purchase is, if it fails, they replace it for free.

I'm going to presume you have a 14,1 iMac, since that's the only late 2013 model with a 21.5" screen.

Crucial's page insists the RAM is not removable for your Mac. OWC lists 8 and 16 GB kits.

If you watch the video on OWC's site on how to replace or upgrade the RAM, it isn't easy, or for the inexperienced. Consider taking it to an Apple Store or other certified Mac dealer and have them do it.

Check out Crucial's web site. They'll tell you what's available for your Mac.

Most of those "wheel" tools are plastic and should not harm the case or the display cover. The real issue is around the camera. There are wires rather close to the edge and a lack of attention in that area could pull an important bit of wiring loose.

Apparently, screen is heavy, you risk of dropping and breaking it, might 2 person job. Your mistake is, you bought used late 2013 iMac with 5,400 rpm hard drive, unless you going be looking at emails, and some web browsing, it will be too slow for use. Also, these 21.5 iMacs are not user upgradable, you should have researched more, or bought brand new iMac with "user upgradable" RAMS.

Canned air dusters can be useful, but you have to be very careful with them. You must use them held level. As soon as you start to tip the can downward, the propellant also comes out. It's very cold, and leaves a residue on whatever you were spraying at.