Don't upgrade.
Your PC is simply too old to make any upgrade worthwhile. Your PC is 7 (not 9) years old. 7 years and it is time to consider a full-replacement. Upgrading such an old PC is not going to get you much. You have a SFF case which limits upgrades in several ways. There is physically very little room and the included powersupply won't support much (a 235watt PSU is below the minimum requirement for almost everything). Even if you found a socket 1156 CPU (this is the type your PC uses) which was faster , then there would be problems with the BIOS needing updating, and maybe your PSU not being powerful enough to support it. Socket 1156 CPUs are now hard to get unless you want a pull (used) one. They are a little costly and not a long term answer. For example, go to amazon and look for 1156 CPUs. If you filter out the used one, they run around 300 or so for a mediocre upgrade.
https://www.amazon.com/Intel-i7-870-Processor-Socket-LGA1156/dp/B002KQ5KEI/
If you must upgrade, consider a graphics card upgrade such as the 750ti ot better still a 1050ti. You would need the SFF version (Low Profile) and you would be best off to not get an overclocked one as the PSU won't support much. Also make sure the one you get does not require auxiliary power, again due to your PSU.
Or consider a new PC.
https://www.amazon.com/MSI-Trident-VR7RC-020US-Desktop-i5-7400/dp/B01NAP5T5V/
https://www.amazon.com/CYBERPOWERPC-GXiVR8020A2-Desktop-i5-7400-802-11AC/dp/B01NBL8BER/
700 to 800 gets you a nice new (entry level) gaming PC.