Peter here from Shopify Support!
Sadly, I have to be the bearer of bad news! Currently, there's no "bulk" way to unfulfill orders that have been marked as fulfilled. To unfulfill the orders that your POS marked as fulfilled will require you to open each order individually and select "Cancel Fulfillment" found in the "More" drop-down menu. I've provided a few screenshots below on these steps in case you need them.
1. Open up your Orders section.
2. Open the order you wish to unfulfill.
3. Select the "More" option (found next to "add tracking.")
4. Select "Cancel Fulfillment."
That should do it! If there's anything else I can help with please don't hesitate to reply back and I'll happily help out!
All the best,