You can delete your Tereo account and everything stored with it at any time. You don't need to ask anyone's permission and you don't need to contact support.
Open Tereo and go to Settings → Account → Delete Account. You'll be asked to confirm. Once you do, the deletion happens immediately.
Email tereosupport@gmail.com from the address your account uses and ask for it to be deleted. Using the account's own email is what lets the request be verified as yours.
This is permanent. There is no undo and no recovery period, so if you want to keep your streak, don't delete the account.
Deleting your account does not cancel a paid subscription — Apple handles billing, and it carries on separately. Cancel it on your iPhone under Settings → [your name] → Subscriptions.
Progress stored only on your device is removed when you delete the app itself.