If you are playing on PC Steam or Windows PC, the additional content for Chapters 3-5 will unlock and be ready to download as an update on October 29. You don’t need to do anything else other than download and install the update.
If you are playing on Xbox Series X|S or PlayStation 5, the Early Release Chapters 1 and 2 download as a separate standalone program. You will still need to download the full game when it becomes available on October 29 in order to continue your save.
No matter your platform, your saves will carry over from your advanced access playthrough. The full game will confirm your saves have been imported when you first open the program.
Completed achievements on consoles will pop when launching the full game on or after October 29. On PC, achievements will unlock as they are achieved, even during Early Access.