You can't upgrade the existing USB slots to USB 3.0, but you can fit a card (like you can a graphics card, good example). Something like this would do it, assuming you have a port like this on the motherboard free:
Because USB 3.0 also provides more power to the device, it looks like you have to plug another power connector into the card as the PCI express port won't supply enough by itself. So you'll need a spare 4 pin connector on the power supply (other cards use different power connectors if you don't have any 4 pin ports spare).
Unless you're doing a lot of copying of data between your PC and external hard drive/memory stick etc, there's not much point upgrading to have USB 3.0 ports. Your memory stick/hard drive will also need to be USB 3.0 compatible in order to run at USB 3.0 speeds anyway.