HP Casefan Replacement
I thinking of changing the casefan but idk the size,the specs of case are here
It doesn't say any information about the case fan, it's most likely to be 120mm, so just get a ruler or measuring instrument and measure the height and width of the fan, it will 99% be 12cm x 12cm because that's what HP most commonly puts in their cases, if it's 12cm then you need a 120mm fan.
The second thing you need to know is what is your fan powered by, look at it's cable, it will either be a small 3-pin cable connected to your motherboard OR a big fat 4-pin molex cable connected to a cable hanging from your power supply.
It will most likely be the 3-pin.
So if my predictions are correct you will need a 120mm fan with a 3-pin connector. Note that you can also get a 4-pin molex fan or a fan with both connectors. All computers have 4-pin molex cables hanging from the power supply.
Note that if you get a 4-pin molex fan, the computer will NOT be able to control the speed of the fan, it will always run at full speed.
make sure that the fan is pointing in the right direction, if it's and exhaust fan at the rear of the case the back of the fan should be facing the back, the fan pulls air out of the case.
Now, decide what fan you want to replace it with, there are huge differences between fans, high performance noisy fans or quiet poor performance fans or balanced fans, they will all be the same cost but if you want a high performance fan that is at the same time quiet, it will be expensive.
Check the flow rate measured in CFM and noise level measured in decibels. You will need at least 40 CFM to keep your PC cool, there are 130 CFM fans too and even higher and for quiet PCs anything below 20db is fine but the quietest fans go as low as 10db, quieter than whispers.