You can change the output voltage of a fixed 3-terminal regulator using a zener diode. The voltage of the zener diode will be added to the voltage of the regulator in order to obtain a voltage of XX + Vz at the output, where XX is the voltage of the regulator, for example 6 V to 7806 (78XX), and Vz is the voltage of the zener diode. A voltage of 7.5 V can be obtained with a zener of 1.5 V. The input voltage must be at least 2 V greater than the output voltage and must not be greater than 28 V above this value. The circuit is shown in the figure.
