Look at the old timer and record the date, start time and duration of watering for each zone. Store this information to use when you set the new timer.
Unplug the sprinkler timer, and open the circuit breaker box. Turn off the circuit to the sprinkler timer. Replug the timer to make sure the power is off, then unplug the timer again.
Using a screwdriver, loosen the screws on the timer. Pull out the timer slightly from the wall.
Remove the wire nuts from the sprinkler, and separate the wires of the timer from the wires extending from the wall. Label the wires with tape so they are not mixed up when putting them back together.
Mount the new timer by connecting the wires. Reconnect the wire nuts, and push the wires into the wall. Attach the timer to the wall with the screws and screwdriver.
Plug in the new timer to the outlet, and turn on the circuit breaker. Program the timer using your notes.