How to Download ECG App on Apple Watch Series 4 and Later
Open the App Store. You can do this by pressing the Digital Crown on the side of your watch and then tapping the App Store icon.
Tap the search button at the top.
Type in "ECG" using Scribble, Dictation, Keyboard or another typing method.
Scroll down through the list of results until you see the ECG app by Apple. From here, you should be able to tap "GET" and it will begin installing.
If you don't see the ECG app in the list of results:
Send this link to yourself via iMessage.
Open the Messages conversation with yourself on your Apple Watch.
Tap the link and it should take you straight to the ECG app in the App Store.