There are several ways to replay adventures:
Adventures with the Repeatable tag: Adventures with the Repeatable tag can be played any number of times, with a different character each time. Unlike other adventures, they also grant a Chronicle every time they are GMed.
Seeker of Knowledge boon: The Second Seekers (Ehu) and Second Seekers (Jadnura) Factions grant Access to the Seeker of Knowledge Boon, which allows a limited number of replays. They are functionally a single Boon; you cannot gain double the replays by gaining the Boon from each faction. See the Boon text for details.
Granted and purchased Replays: All players are granted 2 Replays after their first game. Each January 1st, GMs are granted 1 additional Replay for each Nova they have earned up to that point. Additional replays can be purchased from the AcP Boon Store on paizo.com. Players must have or purchase a Replay prior to completion and reporting of the Scenario they are replaying.
Granted and purchased Replays can only be used on sanctioned content that is Scenario-length or shorter. Each granted or purchased Replay allows one replay of an adventure you have already played as though that adventure had the Repeatable tag.
Replaying for no credit: This is only allowed if the alternative is for the table not to play. Players must record any items or resources expended and can use a blank Chronicle for this purpose. This is an exception to the rule that you cannot assign more than one copy of a single adventure's Chronicle to a given character.