Technically, yes. Mostly no. Certainly not to the extent you can upgrade self-builds, and certainly not with as many effective results.
Really speaking, once a prebuild is built and sold, you can tinker round the edges, or boost some performance metrics. You can add more RAM, sure. You can add a faster SSD. You can upgrade things like your GPU.
But there comes a point of diminishing returns. If you’re going to try and upgrade your prebuild, you have to know exactly which parts you want to swap out, and you have to know both how and why.
At this point, you have to ask yourself why you’ve gone for a prebuild in the first place. You’re dealing with someone else’s design concept, and whatever quality level their components are. You can boost various elements of that design concept by carefully replacing some elements. But be sure you understand what it is about the prebuild that you’re spending money on, and what you want to keep, compared to a scratch-build.