The GX 620 is set up to control fan speed automatically based on the temperature of the CPU. Depending on the BIOS version you have, you may be able to set the Fan speed manually and increase the fan RPM by % or by entering a discrete value (800, 1200, 1400 rpm etc).
To enter the BIOS, press the "Delete" key repeatedly as soon as you power on the computer. This will take you to the BIOS menu. There should be a menu for Power Managment, or the like. You should find Fan settings in this menu. You may need to search around as all BIOS are different. Once you find the fan settings, increase them to your desired setting, then press F10 to save and exit. NOTE....unless you're familiary with BIOS settings, don't mess around with anything else. Just the fan setting.
You could always try to replace your existing fan with a more powerful fan if you can find out what size the current fan is. There are several versions of the GX620 (small form factor or tower, etc), so I don't know what size fan you'll need. You'll have to measure it and then go to a place like Newegg.com and order a fan with the same size, but a strong CFM or RPM rating. Look for something with a CFM of over 60. The fan size is probably something like a 90mm X 38mm.
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