I'm using the FasterCap and noticed that the default unit is m. Is there any simple way to change it to mm similar to what we can do in FastHenry file (.unit mm)?
Actually there is no need. Contrary to mageto-quasi-static impedance, electro-quasi-static capacitance scales linearly with dimension.
So if a structure has m dimensions and results in x pF, the same structure with mm dimensions will have x nF capacitance.
If you still want the result directly in another scale you can simply use a different dimension in the permittivity value, the results will scale accordingly.