Now, scroll down to the Account section, and tap on Delete Account. You'll be asked if you're sure, and if you are, just confirm it! This will permanently delete your Telegram account, so make sure you're ready for that.
If you're not sure, you can always deactivate your account instead. This way, you can reactivate it later if you change your mind. But if you're certain, just go for it and delete it - it's not like you can't make a new account later or anything!
How to delete your Telegram account the right way