How do I enter default value information for my existing entries to a new field added to a sheet?

There are two ways to do this:

1. You can do a mass edit by exporting the sheet as an .xls or .csv file, fill in the new field, and then import the file back if you have existing data. Don't forget to click on Renew data for the Import policy step. Learn more about importing in our documentation here.

2. Write a Javascript workflow program to fill it in.