First of all start by unplugging the unit from the mains supply, otherwise you could get a nasty shock. Making a note of where the wires go- then remove the old timer by unscrewing the nuts. Replace the new timer by reconnecting the wires then tightening the nuts.
If its a new model then they should have appliances at home depot or any hardware store just have to go out and look and be will in to pay a little bit of money for things that are separated from the product. Another great place would be at a Lowe's store.