Ibn Abbas (may Allah be pleased with him) reported that the Messenger (peace be upon him) was cupped while he was fasting - Sahih Bukhari (5694). That is, cupping does not break the fast.
We had the honour of speaking to Shaykh Taha Hanafy this time last year about cupping whilst fasting. He reminded us that only those who are anaemic should avoid cupping whilst fasting. In the case of someone who is anaemic, they can wait for the Maghrib adthan, be cupped and then break their fast.
For those who have been cupped whilst fasting, Shaykh Taha suggested that when the time comes to break the fast, the best thing to drink would be a glass of water with a small amount of pure honey and a small amount of cider vinegar. May Allah reward Shaykh Taha and grant him success.