Can I update an existing integrated Spreadsheet?

Also, we currently are using Jotform for event rsvps. Each rsvp must drop off or mail a check to our office. Is there a way update the excel spreadsheet with check numbers as the checks come in? Or will we have to keep a separate list? 

When you said, "Is there a way update the excel spreadsheet with check numbers as the checks come in?"

1. Did you mean your form is currently integrated with Google Spreadsheet?

OR

2. Are you using the XLS export feature from the Submissions Page?

If it's #1, then no, we wouldn't advise that you edit the integrated Spreadsheet (specially the column titles) as this may break the current integration. It's better to just keep a separate spreadsheet file for you to track those people who dropped off or mailed a check to your office. After all, this requires something that is done outside JotForm and a manual process is required.

If it's #2, then sure, that won't pose any problems. Still, you'd have to do it manually since the drop offs/mails are being done physically and outside JotForm.

In case we misunderstood your question, don't hesitate to explain further so we can better assist you.