Apple stock split posting in yahoo finance

Stock splits are generally automatically handled by stock markets, brokerages, and online tools. (I've never had to handle one manually.)

So the data in Yahoo Finance will automatically be adjusted. But if your Yahoo Portfolio is just a tracking log that you entered your own data into (that is, if it is not synced to a brokerage account), you'll probably have to adjust it. Per this Quora answer:

Prices are adjusted for splits, but your portfolio is not. You have to record the transaction yourself. Your portfolio is not adjusted for rights issues automatically either.

(That's from 2014, so it is possible that Yahoo's Portfolio tools have gained this ability since then.)