update_user.js

update_asset = function(old_asset, updated_asset){  
    
    for (var property in old_asset) {
        if (old_asset.hasOwnProperty(property) {
            old_asset[property] = new_asset[property]
        }
    }

    old_asset.save(function(err){
        //handle possible error
    })
}

...
//then just use like
update_asset(asset,req)