It's difficult to get 5V from a 6V battery because normal regulators (LM7805) require about 2V dropout (to provide a constant 5V, input needs to be above 7V).
To get 5V from 6V you will need a low-dropout regulator (LDO), which has weird output capacitor requirements to be stable, or a switching regulator, which also require external components, and are quite a bit more expensive.