that you must never, under any circumstances tinker inside the cover of a computer's Power Supply Unit. There aren't any user-serviceable parts in there, but there is mains voltage, which can kill both you and the computer stone dead.
The other thing you must never contemplate is soldering anything to the motherboard. One slip will render the board useless.
If you want USB ports at the front of the machine, add them using one of the readily available and very cheap USB cards that fit into a spare PCI slot on the motherboard. You can get one that gives you two rear ports and three front ones at a cost of around £12. At that proce it's not worth tinkering with a soldering iron at the risk of losing a £100+ motherboard.
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