UARFCN configuration command for 3G

I have faulty RRU 3936 carrier 3g 850mhz and have spare RRU 3936 but 900mhz RRU. I have successfully installed and currently site is up.
But, now I need to change the UARFCN from Huawei U2020. What command do I have to use to change the frequency?