how can I auto delete old submissions

Though it is not possible to automatically delete old submissions, you can delete single or groups of submissions manually.  You can do that by following the screenshot below.

1. Navigate to your "My Forms" page->Choose a form you want to clean up the submissions->Click the "Submissions" option.

2. Under the "Submissions" page, you can:

A. Delete specific submissions.

B. Delete specific list of submissions by setting up a custom date range where you can view and delete them.

C. Delete ALL Submissions

3. When you scroll down you can see the delete submissions button. I would recommend that you download all your data so that you can have a back up on them, please take note that once you deleted your submissions you can't recover them back. This is the same with uploaded files, you need to download them first so that you can have a copy for future use.