The Summer I Turned Pretty (TSITP) is available to watch by streaming on Amazon Prime Video. All three seasons, including the latest episodes, are currently available on Prime Video with a subscription. There is also an option to watch with ads on Prime Video Free with Ads, but there are no completely free streaming options otherwise. The show is a Prime Video original, so a subscription to Prime Video is required for full access.
Some unofficial sites were mentioned in discussions, but the recommended and legal platform is Prime Video where the show is regularly updated, including the latest season releases.
In summary, for the best and safest experience, watching TSITP on Amazon Prime Video is the correct option.