Can users safely update their existing information when they access the form?

Can I upload existing user information to JotForm so that when users access their individual form they can see their existing information and update it? If date does not exist for a user, they can add it. 

You can use the Import Tool in order to import a CSV or Excel file and combine it to the existing form submissions. Please refer to this guide: Import-App-Easily-Import-Your-Excel-or-CSV-Data-into-JotForm.

You may also use the Spreadsheet to Form widget. This widget allows you to upload your spreadsheet to your form and use the values you have on the spreadsheet to pre-populate your form. Please check this guide: How-to-Use-the-Spreadsheet-to-Form-Widget.

Hope that helps. Thank you.