When I upgraded my RAM, I think Apple only offered an upgrade from 2 to 4 GB. I wanted more. I ended up buying 8 MB upgrade replacement RAM (physically this is just two small circuit boards) from a company called OWC (
https://eshop.macsales.com/shop/apple/memory/ ). Their website walks you through exactly how to find the RAM you want, provides reasonable prices and videos on how to install.
For example,
this link shows that I can buy 8 GB of RAM today for my machine for $67. And found
this link which has a video on exactly how to install the memory. It was easy to do and took only about 15 minutes. If I wasn't comfortable doing this, I could easily have found many friends (or even my kids) that could identify the right RAM to buy and then follow the video easily to install.