as already mentioned, without strong knowledge in electricity & electronic, I would advice you to not even try because of real danger during reparation and after.
Now I see maybe one exception for this rule: if it is only a fan related issue. If it is just your fan that stopped turning but your psu still work, replacing the fan is imho feasible.
I have encounter the problem where the fan stopped working but gpu still fed. I used an external fan controller to which I connected the fan and this psu is still functioning right now.
but tbh if I could I would have bought another psu. for the moment all is good and I hope it will still be ok
Yes, related to the fan, I also agree to open it and unplug the old fan and then put a new one, you dont need to put an identical fan, put another one, need to be a strong fan here, at least 2000 rpm on top of it to make it blow the hot air away and cool down the psu components, how do you know if is the fan? The safety circuit will trigger if the temperature is higher than allowed, meaning your psu will turn off, that is called "The Overheat Protection (OHP)".