[{"_id":ObjectId("60e46c1dfc320656ffc683b4"),"name":"A","name_test":"a b c","arr":["1","2","3"]},{"_id":ObjectId("60e46c20fc320656ffc683b5"),"name":"B","name_test":"B A","arr":["a","b","c"]},{"_id":ObjectId("60e46c26fc320656ffc683b6"),"name":"T1","number":1,"name_test":"T1","arr":["t1","t2","t3"]}]

db.collection.update({$expr:{$gt:["$arr",1]}},[{$set:{first:{$first:"$arr"},last:{$last:"$arr"}}}],{multi:true})