TENS modification

i need a way to change frequency in the 0-150 Hz range, the arduino pwm

Just google:- arduino pwm frequency change

i also need a way to change voltage and current

No the voltage output from the arduino is a fixed logic level. The current output depends on the load.

here is the diagram of the circuit as it is now.

Thanks

as you can see i have .......

Sorry I can't make head nor tale of that schematic. Too many loose ends, what is connected to what?