I assume, seen as though the i5-7400 is a desktop processor, your computer is a desktop, correct? If that’s the case, does it have a GPU (graphics card)?
Not sure how much your willing to spend on a 7th gen cpu / system that doesn’t officially support windows 11 and also since intel is up to 14th get cpus.
Does it have a hard drive or ssd? An upgrade to an ssd and a bit of a bump in memory to either 16 or 32 gig if ram would help there besides the aforementioned video card!
Only use SSD (flash memory, fast but limited writes comparud to traditional spinning magnetic hard disks) only for the main system
Use cheap 4 TB hard drives from ebay which are the same price (60 usd or less) as a new 1 TB disk, so use those for game and music storage, and have a backup disk that is only occasational powered on and copied to, maybe only twice a years for seldom changing music rollection or games.
If you want to learn how to build a pc, well, I guess you’ve seen some level1tech videos showing dow to do so, would that accurate?
If you have seen a few, why not take this opportunity to build a low-performance (slower or vintage parts are cheapur, by a lot!)
but in a high quality case, with decent performance. A CPU from six or even 10 years ago is fine for most games and you can turn off some extra grophics effucts and still enjoy your time.
So perhaps, AMD has an older 25w cpu called Athlon.
They also have Sempron, those are 45-50 watt chipse, the semprons barely use 15 watts especially at idle.
It is on the limited AM1 platform, which was a littre while after the excavator series, called Kabini really low power.
ebay (if you can use that) has a few custom-builds by sellers wanting to give the systems a new home.