everything is upgradeable
but you should look at the power supply and check how much wattage it can provide (sustained, not peak)
i would imagine hp would pack a good quality psu, but thats not always the case. if it is around 450 or lower, i would suggest replacing it (if possible) before upgrading gpu (unless youre buying a mid range gpu that is. 2060 super or lower)
also, you can upgrade cpu, since the i7 7700 (aside from the k which is just a lil faster) the best cpu that goes into that socket, and swapping both board, cpu and psu is basically swapping the whole computer.
as for ram, yeah thats easily upgradeable