I don't know how you would go about recalibrating it. I do know that it is susceptible to some errors.
The temperature compensation is not good. At cold temperatures, it overreads even with the built-in temperature compensation turned on. About +0.5 UVI at 5degreesC at midnight (actual UVI should be zero then). The overreading seems to be getting worse over time.
It also gets reflections from the window at certain sun angles. I'll get a spike in mid-morning and mid-afternoon just from the internal reflections. There are several reflections. I have not identified them all, but some are off the sides of the window.