If you upgrade your account in the Reminders app on a device with iOS 13, iPadOS 13, macOS 10.15 or later, you see upgraded reminders on iCloud.com. To see which version of Reminders you have, look for one of the following:
If you’re using upgraded reminders, you see at the top-right corner of each reminder list.
If you’re not using upgraded reminders, you see Options at the top-right corner of each reminder list.
After you upgrade, you can view your reminders on iCloud.com and on your devices with iOS 13, iPadOS 13, macOS 10.15 or later that are signed in with the same Apple ID. If you also have devices with earlier versions of iOS or macOS, you don’t see your upgraded reminders on those devices.
See the Apple Support article Upgrading the Reminders app in iOS 13 or later.