delete data from postgres database using heroku (ruby on rails)

I am developing an website with heroku (ruby on rails). I use the postgres database. Now I made a mistake and there is a row inside this database which I want to delete. But I have no access. The data is shown on the website. But there is no picture or other content. It was just a trial and a big error ;)

How can I delete this data without to delete the whole database? I found nothing...

I hope you can help me.

with best regards