How to change a submission or add a new submission?

Also, can you tell me how to change a submission or add a new submission?

You can Edit the Submission via the Edit link that you can add on the Email notification you will receive when your form is submitted.

Then, on this window: 

So when someone submits your form, you will receive an Email notification with this link:

You can then update the values in the form fields and submit the form again.

Remember that the submissions stored in your submission page will also be updated after submitting the edited form. So if you want to keep the old submissions before it is edited, you can save those submissions first by downloading a PDF, CSV or Excel copy of it. 

How-to-Partially-Download-Form-Submissions-as-Excel-CSV-PDF

You may also check this guide for reference: How-to-Let-Users-Update-Their-Form-Submissions-at-a-Later-Date

I hope this helps. Let us know if you need further assistance. Thank you.