You can playtest the next edition of Pathfinder by playing Doomsday Dawn. It's broken up into 7 distinct chapters. Give it a whirl and then give Paizo your feedback. They are using surveys to collect your thoughts, so taking part in the playtest is very easy. Player feedback has already had a profound effect on plans for the game. The links to Surveys are all on this webpage.
You will want to download the free Playtest Tracking Sheet PDF that will help you keep track of a few important pieces of information that you need to fill out in the surveys. There are surveys for players and surveys for Game Masters. There is also a general feedback survey for each part of the Doomsday Dawn adventure, so you can give Paizo your overall reactions. Links to all the free downloads are all on this webpage.
Even if you are not playing Doomsday Dawn, you can still give Paizo input! There are a number of general surveys that ask questions about a wide variety of topics in the game. They focus on the various classes, ancestries, and play mechanics. You can also engage with other players and designers over on the Playtest forums.
There are four Pathfinder Society adventures which give you even more opportunities to play. Create your own character or grab a pregenerated character to play these. Or, bring a 10th-level character of your own design to face an even mightier challenge for a higher level game! For each playtest adventure you experience and report, you will also earn Playtest Points which can be redeemed at the start of a new Pathfinder Society campaign in August 2019. The links to Surveys are all on this webpage. The forum on the Playtest is here.