simulate this circuit – Schematic created using CircuitLab
Figure 1. a) 5 V regulator. b) Boosted regulator.
The 1N4148 isn't a Zener diode - it's a regular small-signal diode made of silicon and capable of about 100 mA or so. It has a forward voltage drop of 0.7 V (capital V for the symbol).
The 7805 regulator works by adjusting the output voltage to 5 V above the voltage on the "GND" pin. That means that if we can hold the GND pin at some other voltage then we can adjust the output voltage.
The diode works because the regulator passes a few milliamps out on the GND pin. Thi is shown as 4 mA on the simulation of Figure 1a. If we run this current through one or more diodes the effect will be to raise the GND voltage by the diode's forward voltage drop at that current - 0.645 V according to the simulation.
VM4 shows that the GND voltage has been raised to 1.29 V by the addition of D1 and D2. This results in the regulator's output being raised to 5 + 1.29 = 6.29 V.
All of the 78xx series regulators work in a simular fashion so the 7809 output voltage can be raised in a similar fashion.
simulate this circuit
Figure 2. Using a Zener diode to raise the output voltage.
Note the different symbol for the Zener diode and its reverse connection.
The capacitors are for stability and prevent oscillation of the regulator. Typically 10 nF will do the job. I've shown a 10 μF on the input but a smaller one can be used if the input supply is stable.