db={branch:[{"_id":ObjectId("62c447e3cf7aee7b78d39908"),"gid":"1234","branch_trans":[{"branch_id":"branchId123","op":"test_op"},{"branch_id":"branchId456","op":"test_op"},{"branch_id":"branchId789","op":"test_op"}]},{"_id":ObjectId("62c447e3cf7aee7b78d39909"),"gid":"1239","branch_trans":[{"branch_id":"branchId129","op":"test_op"},{"branch_id":"branchId456","op":"test_op"},{"branch_id":"branchId789","op":"test_op"}]},{"_id":ObjectId("62c447e3cf7aee7b78d39905"),"gid":"1235","branch_trans":[{"branch_id":"branchId125","op":"test_op"},{"branch_id":"branchId456","op":"test_op"},{"branch_id":"branchId789","op":"test_op"}]}]}

db.branch.aggregate([{"$replaceWith":{"$setField":{input:"$$ROOT","field":"branch_trans","value":{"$concatArrays":[{$slice:["$branch_trans",0,1]},[{"branch_id":"branchId456","op":"op1"},{"branch_id":"branchId789","op":"op2"}]]}}}}])