Can I replace the lens on Pixy2?

It depends on which version of Pixy2 you have. Pixy 2.1 has a replaceable M12 lens, but Pixy 2.0 does not.

You can use practically any M12 lens with Pixy 2.1 as long as it has an IR-cut filter. (The IR-cut filter is something all CMOS sensors need to eliminate most of the ambient IR light, which can adversely affect the color fidelity.) Pixy 2.1 ships with a 1.7mm focal lengh F2.0 lens, which gives it an 80-degree horizontal field of view.

Pixy 2.0 has a slightly smaller field of view - 60 degrees horizontal.

Engineering is all about managing a series of tradeoffs!