As I understood you want to update field values already stored in your DB, here are some options :
1 - Delete the whole Database from Deployment folder “data”, a new database will be created once you run your app.
2 - Update from the Database manually ,while running your app under console “advance > start built in database viewer” , find your table and perform an update statement .
3 – Update using a Microflow , Create a microflow to retrieve all the records of that entity , loop and update then commit .
I hope this is what you need .