If you received an email asking you to download the new app for one or all of your saved addresses, then you need to download the new all-purple ‘Ocado: Online Supermarket’ app.
For the App Store you can download it here: https://apps.apple.com/gb/app/ocado-online-supermarket/id1552379398
For Android you can download it here: https://play.google.com/store/apps/details?id=com.ocadoretail.mobile.android&hl=en
Your password will be the same as usual, so nothing has changed with that.