Once you have completed the online check-in process with Ryanair, you generally cannot change your passport or travel document details through the website or app yourself. The system "locks" your boarding pass information once it has been generated. However, if you realize you made a mistake—such as a typo in the passport number or an incorrect expiry date—you do have options. For minor corrections, you can often use the Ryanair "Live Chat" feature on their website; a customer service agent can frequently update the details for you remotely without charge, provided the name on the ticket remains the same. If you wait until you arrive at the airport, you can visit the Ryanair ticket desk, but be aware that they may charge an "Airport Check-in Fee" or an administrative fee to re-issue the boarding pass with the corrected information. If your name is completely different (e.g., due to marriage) and doesn't match the passport, this is treated as a "Name Change" and carries a significantly higher fee. It is always best to double-check your details before hitting the final "Check-In" button, but if a mistake happens, contacting their support team immediately is the most cost-effective solution.