Custom ROMs are not official firmware. Proceeding comes with caveats:
Years ago, Huawei allowed users to unlock their bootloaders via an official website. However, they discontinued this service. Today, obtaining an unlock code for a Huawei Y6 Prime is difficult. huawei y6 prime custom rom
The process involves a factory reset, which will wipe all internal storage. General Installation Process Custom ROMs are not official firmware