Depending on the fan, it shouldn't be difficult. Most fans come in standard sizes, usually 80mm or 120mm. It's usually just a few screws and a small 3 or 4 pin wire lead that attaches to the motherboard. Just open the case and take a peek.
Unfortunately, if that's the only fan to the outside of the tower, it's most likely the power supply fan, in which case it's safer to just change out the power supply.