Before replacing the fans try to clear out the dust.
Over time dust gathers around the fans and heat sinks and can block up the air vents.
If the vents are blocked up the fans have to work harder to keep inside the case cool.
Take the PC case outside, take off the cover and carefully blow out the dust with a hair drier.
Dislodge the dust CAREFULLY with a small piece of wood (a pencil or lollystick or similar - DONT USE METAL) or brush it off with a small CLEAN paintbrush.
Blow out all the dust you have dislodged.
Make sure you have not dislodged any cables or cards.
Before you put the cover back on plug in the PC unit and make sure the fans are still going round.
When you are happy put the cover back on and reconnect everything.
Make sure there is plenty of room round the PC case and that air can circulate. Dont box the PC case in tightly so there is no room for air.