First things first: update your messenger app to the latest version. On the Google Play store page, it will show what’s new.
Next, hold down the heart reaction on any message on your messenger app for 2-3 seconds. This message should pop up:
Now you can use the new heart reaction!
If you don’t want to use it, just hold the heart reaction again then this message should pop up:
Will you start using the new heart reaction? We definitely will!