First, update to the latest bios version, if you are not already on it.
Second, reset the CMOS settings in the BIOS by powering off, unplugging the cable from the power supply, removing the CMOS battery for five minutes and during those five minutes press the power button on the case for 30 seconds. After the five minutes, reinstall the CMOS battery, plug the power supply back into power and power on the system. Go into the bios and check to see if you are able to now make changes to the CPU multiplier.
It's likely you might need to change the AI overclock tuner setting to manual or XMP, and the core settings to per core (Or synced, but I like per core much better as a granular approach that doesn't force cores to full frequency when they don't need to be, giving them time to rest and extending the lifespan and reducing package temps) before changing the multipliers.