If you have never made any payments on Tilda, you can delete your account following this link: tilda.cc/identity/deleteaccount/. Type "DELETE" and enter your password. Please delete manually all pages that you created.
If you have made payments on Tilda, contact our Support team.
Alternatively, you can switch to a Free Plan and delete your projects. We do not store personal data. The Free Plan does not oblige you to do anything, yet keeps the opportunity to come back on Tilda and new create projects.