Hello,

Go to File>Export>Vendors and create an export template that includes the vendor number, EFT enabled and whatever other EFT fields you want to update. Use the exported Excel sheet to mass update the information and then go to File>Import>Vendors and set up an import template with the vendor number and the fields you're updating and use "overwrite records".
Imports are finicky so you must observe the field formats and should do a backup of your data before doing any import to master records.