Greetings, and thank you for your question.
There is not really a direct conversion from Xanax (alprazolam) to Valium (diazepam) because Xanax is short-acting and Valium is long-acting.
Xanax 2 mg tid (three times a day), for a total dose of 6 mg, is pretty hefty, so crossing from one medication which is short to another that is long will probably result in withdrawal-like symptoms. A cross taper, under direct guidance and supervision of a physician, should occur.
The target dose of Valium, coming from 6 mg of Xanax daily, should probably be in the 30 mg daily range, at least, divided into bid or even tid dosing.
Hope this helps to get you started! Please let me know if you have any further questions.