Why can't I get higher voltage than 3.75V on Arc?

The maximum output voltage level depends on how you have connected the Arc and in what mode you are using it in.

The best way of increasing the maximum output voltage is to plug in a DC adapter on the backside of the Arc (this increases the maximum output voltage from 3.75V to 4.55V).

How do I choose the right external DC adapter for Otii Arc Pro?

You can also change setting from Auto Range to High range (this increases the maximum output voltage from 3.75V to 4.2V) The drawback with this is that the measurement for low currents are not as accurate as in Auto range mode.

If you both change to high range mode and plug in a DC adapter, the maximum output voltage increases from 3.75V to 5V