How can I increase the base-emitter voltage of an NPN transistor in forward active region?
I think what you want is to control ratio of voltage base-emitter and emitter-collector.
You can do it by changing ratio of C2 to C3, keeping the total the same.
If you want higher voltage output of the circuit, you need to make C3 smaller and C2 larger. This will also make startup and stability worse though.
I want to add that such circuits in general are probably not a good start. They can completely change behaviour somewhere else when something is changed.