Specific Realm database values must update

Question

I'm implementing Realm in React Native. The context is that I've 2 schemas declared on top ie 'PortfolioData' which has a type of schema 'PriceAChangeSchema'.

Now on my update screen, I've to update just 3 values in PriceAChangeSchema ie balance, lastPrice and totalVal. The thing when I try to push just 3 values inside the realm write function, it successfully overwrites the 3 values but the rest of the values of PriceAChange schema becomes null which I don't want to.

PS The values of PortfolioData upon pushing remains the same but the PriceAChange schema values becomes null. ?

class PriceAChangeSchema extends Realm.Object { }

PriceAChangeSchema.schema = {

    name: 'PriceAChange',
    properties: {
        icon: 'string?',
        cryptoName: 'string?',
        balance: 'string?',
        lastPrice: 'string?',
        change: 'string?',
        totalVal: 'string?',
    }
};

class PortfolioDataSchema extends Realm.Object { }

PortfolioDataSchema.schema = {

    name: 'PortfolioData',
    primaryKey: 'select',
    properties: {
        from: 'string?',
        select: 'string',
        apikey: 'string?',
        secretkey: 'string?',
        address: 'string?',
        logoLink: 'string?',
        priceandchange: 'PriceAChange[]'
    }
}

realm.write(() => {
            const ExchangeIns = realm.create('PortfolioData', {
                select: select, //primary key
                priceandchange: [], //want to change specifics
            }, true);

            ExchangeIns.priceandchange.push({
                balance: '0.004',
                lastPrice: '2.22',
                totalVal: '3.5',
            });
        });