[{"_id":"60d3fcb6077b0a41e090ecc9","vendor":"Best","test_name":"Number Sequence","discipline":"early math"},{"_id":"60d3fcb6077b0a41e090edd7","vendor":"Best","test_name":"Value Places","discipline":"early math"},{"_id":"60d3fcb6077b0a41e090ecc8","vendor":"Best","test_name":"","discipline":"early math"}]

db.collection.update({"vendor":"Best","discipline":"early math"},[{"$set":{"test_name":{$concat:["EM: ","$test_name"]}}}],{multi:true})