Can i downclock my cpu to reduce heat of cup and interior of casing

The most important thing is to fix your problem so what have you tried so far to improve cooling of your CPU?
1) downclock reduces heat
2) reduces performance
1 stick will reduce heat but hardly noticable.

so do i need to imporove the ventilation of my case?

other computers in the same room with my pc has about 35C and my pc is always above 45 at idel with even monitor turnoff in idel mode and max about 75

Ventilation helps but have you cleaned the CPU heatsink and fan? If so have you reseated the cooler with new thermal paste? Those are the first steps in troubleshooting CPU heat problems.

i've clean the cpu head sink and fans with a strong blower.
my mobo temp is also high (normal 45C and max 70+.
and my 4550 fanless is 50C-70+.
so i think i need ventilation inside my case. so plz let me know ur opinion
i've one blowing 80 mm fan inward and power supply fan outward

Improved ventilation will help, having a fan less GPU requires better ventilation adding an extra fan for exhaust would be my first step.

ok, i'll buy 1 or 2 fan for ventilation
but let me know how to downclock the cpu.

thanks

Reduce fsb in BIOS.

You shouldn't need to reduce your cpu speed really, what case do you have?

Thermal paste - http://www.arcticsilver.com/methods.html

Exhaust fan under graphics card - http://www.newegg.com/Product/ProductList.aspx?Submit=E...

^^^

Yeah, maybe a fresh coat of thermal paste will fix the problem.

You'll need to remove the heatsink from the CPU, then clean original thermal paste from the heatsink and CPU with rubbing alcohol and a lint free towel. The apply thermal paste (about the size of a rice grain) to the CPU and spread it evenly with something like a credit card or playing card.

china made cheapest casing, i think
it's my work computer provided by the office

Does it still work or is it having performance problems? If it is having problems do you have a IT department?

Myo: my phone battery empty twar p
switch off htar dae
yesterday ka my bed side hmar yu htar lo charge ma lote lite ya bu

Ok.  

Obviously....  

sorry
wrong post