Yes, you can replace the fan - Acurite sells a "AcuRite Iris™ (5-in-1) Wind Cup Assembly Replacement" - it's $20 and it includes the solar powered fan as part of the assembly. All you will need is a philips screwdriver to replace the assembly (just keep the screws together).
https://www.acurite.com/shop-all/weather-instruments/weather-sensors-and-parts/replacement-parts/replacement-5-in-1-sensor-assembly-wind-cups-5n1wcbsa.html?ref=name
They will include a step-by-step replacement guide with the new part - it's really intuitive.
BONUS - once you have it open, check and clean out any dirt/debris that may have accumulated + you can clean the top half of the sensor while you have it apart.
If you are handy with a soldering iron and a multimeter I would suggest you check the solar panel and motor before replacing it - you might find there's a broken connection (Acurite engineered this to be modular - the solar panel + fan is an isolated circuit, you can remove it without affecting the other weather sensors).