Your best bet would be to drop the overclock back abit until the cores are not going above 74c when running prime just to be safe for now concidering your using the stock cooler.
I would suggest saving the money up for a decent cooler like the ones mentioned above as they will allow you to oc further and keep it allot cooler than it currently is which will in turn pro long the life of the chip.
If you are not confident enough to change the cooler yourself, and apply the thermal paste then if you know someone who has done it in the past or you have a computer shop near you, then you could always ask them to do it for you.
Putting the coolers on the cpu is one part that I hate myself, and I will normally do a dry fit 1st to see how it goes and then I will add the thermal paste and test it once up and running, if I think the temps are a little high I will take off the cooler and see how much paste is left or not left and if I used to much I will clean it off and re apply using less or if I didn't use enough I will do the same but with more paste.
It is really trial and error when you 1st start changing coolers yourself if you have not done it before.
I am 26 years old and have been building pc's since I was 10 years old, and I up until this year would always get my dad to do the coolers or waterblocks, simply because the 1st time I tried it I cracked a board, 2nd time I didn't put enough paste on, 3rd time I put to much and now I am starting to get the right amount each time.
There are plenty of videos and guides around the net on how much thermal paste to use and fitting guides etc and I would suggest watching them before you try it yourself and listen to what, the people say because some coolers are so big that they are difficult to fit.
Either way which ever you choose to do good luck and welcome to the world of pc's once your bitten by the bug your bitten and it never let's you go
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