Does anyone know how I can reset

Printed Picking Ticket

to 'F' for a transaction?

Not really possible. Per SA 23971, "Only creation of Item Fulfillment for partial fulfillment can revert back Printed Picking Ticket field value to F."

You should be able to check the "allow reprints" on the Print/Checks Forms -> Picking Ticket and reprint a Picking Ticket if you need.