For anyone struggling with how to use this command, here is a working example:
update_asset METAFEES null {"max_supply":"12000000000","market_fee_percent":30,"max_market_fee":"1000000000000000","issuer_permissions":79,"flags":128,"core_exchange_rate":{"base":{"amount":1,"asset_id":"1.3.0"},"quote":{"amount":160,"asset_id":"1.3.472"}}} true