Since version 18.1 of the iOS app, you have the ability to close your account in your Account Settings.
To do so, tap on your profile image in the top right corner.
Then tap Account Settings.
Then, you can permanently close your WordPress.com account from the bottom of your Account Settings screen.
If you’re not using the iOS app, you would need to log in from a browser here:
Once you do that you can follow the steps on the link below:
Please note that deleting your account will delete all sites associated with it, and the username is blocked from being registered again.