Take the lid off an have a look, or if you can't be bothered to do that see the photos in https://wiki.hummy.tv/wiki/Hardware_Information
where you will see the principal components are labelled. It ain't like a PC where components are fitted in sockets (at greater expense and increased rate of failure)!
Interestingly, there is a spare footprint on the PCB that looks like it could accommodate another DRAM chip, but it would be an extreme challenge to actually fit one - they look like BGA (ball grid array, where the contacts are inaccessible underneath the chip and only solderable in a reflow oven). But even once fitted, you have no idea whether the operating system is set up to expand into it, or whether the performance would improve if it did.
(I could have just said "no".)