Short Answer:
Not possible via Magento2 Admin Backend.
Solution:
You need to update eav_attribute table and edit information about backend_source, frontend_input etc.
Simple query for that is:
UPDATE `eav_attribute` SET `backend_model`="Magento\\\Eav\\\Model\\\Entity\\\Attribute\\\Backend\\\ArrayBackend", `backend_type`="varchar", `frontend_input`="multiselect", `source_model`=NULL WHERE `attribute_id`=YOUR_ATTRIBUTE_ID_INTEGER LIMIT 1
Replace YOUR_ATTRIBUTE_ID_INTEGER with your attribute_id like 355