According to the documentation “Data-Only Blueprints are edited in a compact property editor” but for the life of me I can’t find it. Editing values in the standard “Defaults” panel is less than ideal when there are many variables and only a few are meant to be edited (marked as public and editable).
One, simpler way to edit defaults is placing an instance of the blueprint on the map, editing properties in the Details panel then clicking on the Apply button in the Blueprints section of said panel.