Deleting a Twitter account from iPhone Twitter app will not delete or deactivate the account. However, it will delete the account from your iPhone. If that's what you want to do, then follow the simple steps below. If you want to delete a Twitter account completely, read the instructions on wikiHow for deleting a Twitter Account.
Step #1. Open the Twitter app on your device
Step 2. Log in with your username and password
Step #3. Tap the button at the bottom, on the right side of the app
Your account information will be displayed.
Step 4. Click the "Accounts" button
It is an image of two silhouettes.
Step 5. Choose your account
Slide to the left so that the "Delete" button appears.
Step 6. Click the "Delete" button
You will then be redirected to the login window.
Step 7. Check carefully whether the account has been deleted
Go to the Home screen and open Settings. Open the Twitter app. The username and password should both be empty.
Step 8. Sign in with your other Twitter account
Have fun tweeting!