well the i5-4440 is a decent enough system. with some upgrades it can run PUBG @ 50-60FPS.
Cleaning:
===========
cleaning: disconnect power, open case, and blow air (canned air) into slots, cracks etc., wear mask it will blow back into face, and get the "top layer out."
then it will be a process of getting down with the dirt, you may have to pull the fan over the heatsink and get more of that dirt out from fins. wipes fans down with moist alcohol wipes or moist "Windex" cloth (removing crud from fins of fan fins)
important and maybe obvious but
"never spray any product i.e. liquids of any kind, into/unto the hardware components"
I use air can or when big problem, a special vacuum (electronics vacuum)
terry cloths (paper rips and leaves more deposits than removes dirt)
99.99% Electronic Alcohol (70% is not good at all and leaves deposit on boards and alcalines that can cause shorts)
Old tooth brush (previously cleaned and dried) to get in corners, scrub fan fins, reach into motherboard slots.
"window cleaner type product" to clean inside panels of case and such non electronic areas.
Gluck and do a picture of before and after, its amazing how rewarding it can feel
Upgrades and issues;
===============
As for making it into a gaming machine your first hurdle will be your power supply, this will absolutely be limiting what video card you can use. the main question in this regard will be is the computer a slim model desktop or a normal mid tower model ?
if the later then you should be able top replace it with a more power PSu and then upgrade video card.
The next upgrade will be ram, 6gb is not sufficient, and depending on your desktop model (read me the product label on the side of case) may or may not be able to be upgraded, if you can, then I would push for 8gb or 16gb...
the 2tb hard drive unless it is a 5400 rpm drive could be reused, again upgrade wise would be a consideration of reformatting it clean, then adding an SSD ( 60GB/120GB are pretty cheap) for the new OS install and keep the 2tb for storage.
then finally the choice of video card.... : )
so depending on your budget.... this can get expensive quickly.