Push the front button in the front to open the tray, if it does not work (no lights/power) - you might need to open the computer and re-plug the SATA power and data cables inside.
it should be able to handle i7 7700/7700k based on support.hp.com/us-en/document/c05956608 But I would watch CPU temperature while using extensive software or gaming. I read that temperature could go hot and shut down the PC.
I found this - www.reddit.com/r/sffpc/comments/164kj9b/hp_elitedesk_800_sff_most_gens_cooling_upgrade/ There is an entry on the link where you could get a compatible cooler from ebay for 11 dols.
Yes, you may upgrade to i7 7th Gen. Just keep in mind that the motherboard only supports up to 7th Generation. Windows 11 requires 8th generation and above, however there are work around to bypass TPM and Processor requirements - google.
