If your form is nearing its submission limit, you can clear the submission data by clicking 'Clear form data' under the 'Editing and Publishing' section in your form's settings page.
Before you clear the data, it is best to export it first, so you have a copy. Once you clear the data, you can not get it back so it's good to make a copy of it first.
Clearing your form data will delete every submission and reset the analytics for this form.
This action cannot be undone, so be careful!