First identify the entry then just add another integer to the value.

It depends on how you are updating that object now - if it's always in 0 position then just portfolioHoldings[0][amount] += number;

if not, then loop through all and check if ticker_symbol == AAPL, then update that array value, by adding needed amount.

I wouldn't want to assume that "AAPL" is always within the array.

Check first if the new item is within the array. If not, I will return and end the call (but you may wish to append this to the array instead as it is a new item).

If it is, replace the current object (or parts of the object, in this case I have done just fees & amount)

function addPortfolioItem(item) {
    const index = returnPortfolioHoldingIndex(item);
    if (index = -1) {
        // Returning if not found - You may wish to append to the array as mentioned above
        return;
    } else {
        // Overwrite the object within the array with the new object
        portfolioHoldings[index].fees = item.fees;
        portfolioHoldings[index].amount = item.amount;
    }
}

function returnPortfolioHoldingIndex() {
    const tickerSymbolToSearch = "AAPL";
    return portfolioHoldings.findIndex(item => item.ticker_symbol === tickerSymbolToSearch);
}