Upgrade in what way? More memory, newer processor, larger HDD/SSD?? We can only speculate with the information provided. Many Chromebooks are under $400 and most new components may be a higher cost. If your current device is no longer suiting your needs/requirements, it is usually cheaper and easier to buy a new system than trying to upgrade internal components.
It’s not the OS, that should be upgraded, it’s the maker, and it might be different for different models and different makers. So this is a question that can’t be answered.
– What does the tech manual say about it?
– What does the customer service of the shop say about it?
– What does the customer service of the maker say about it?