QuickBooks Online currently doesn't have a direct bulk-editing feature for customer details, Essa.

If you re-import updated customer data, QBO won't  update existing records. Instead, it'll create duplicates.

Given that you handle a large number of modifications, like correcting 250 customer records with incomplete information, you'll have two options to consider: you can either make your already imported customers inactive before re-importing the updated list, or edit each one individually.

After making your existing customers inactive, you can now upload your updated file. From there, QBO will import the new data without creating duplicates, allowing you to efficiently update your customer information.

Feel free to reply to this post if you need further assistance.