Looking at those specs it would not be easy to slip in a 3ghz processor. you have other problems which are too numerous to go through. Basically this system is only good fro surfing the web, email, etc. Playing a modern game is not within its' realms of operation. I am afraid it is a new pc, but if you do go this way, be careful before you buy and search these forums for people to stay away from. Market leaders to look at Ebuyer, Amazon, Aria technologies, Scan and Novatech. All of these are reliable suppliers. They will have PC's aimed at the gaming market, but you will be looking at the thick end of 600/700. if you are unsure post the specs here and someone will advise you.
OnePlus 5T review: Hands-on