Hi ! You can simply use .to_json() - see documentation here
Here is an example using SQuAD:
from datasets import load_dataset
squad = load_dataset("squad", split="train")
squad.to_json("squad.json")
data_files = {"train": "squad.json"}
re_squad = load_dataset("json", data_files=data_files, split="train")
This creates a JSON Lines file, then it reloads it using the JSON dataset loader