[{_id:"id1",_cells:[{k:"key2",v:"value2"},{k:"key1",v:"value1"},{k:"key3",v:"value3"}]}]

db.collection.update({},[{$set:{new_field:{$concat:[{$arrayElemAt:["$_cells.v",{$indexOfArray:["$_cells.k","key1"]}]},"_",{$arrayElemAt:["$_cells.v",{$indexOfArray:["$_cells.k","key2"]}]}]}}}])