Are you using a computer?
From your "Build Registry" page, look for the tiles under "Finish Your Registry Setup." You may have to click on "See All" to access the correct tile. 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?
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.