I was on .5 mg xanax (alprazolam) for about three weeks once a day for sleep then 1 mg a few days can i just stop?

Yes, you can: Better, gradually taper the Xanax (alprazolam) down to zero. The manufacturer's recommendation is 0.5 mg at a time, every 3 days. Good luck.

Possibly...: I would recommend cutting down back to 1/2 for a night --> 1/4 for a night --> stop. In actuality, you can discontinue it immediately since you were only taking it at night (from a physiological withdrawal stand point), but I recommend the brief taper.