Are you using a computer?
Log into your account on the website. Look for the "Build Your Registry" tiles at the top of your registry (above your gifts). Click on your arrival date tile and a pop-up will load where you can edit the date.
To update the shower date, click the "Shower, Sprinkle or Just a List?" tile to add or edit the date.
Are you using the app?
In the iOS App:
Tap the menu bar in the top left corner and select "Registry" then "Arrival Date." The shower date can only be updated on the website.
In the Android App:
Hit "..." and then "Edit Registry". Scroll down to your arrival date and tap it to edit it. The shower date can only be updated on the website.