I'd go back to cure my pontentially fatal heart promblem so I'll never have to live with. Bottom line, I'd cure my heart condition by showing medical evidence.
I would go back in time to destroy DSiPaint because I used the said time machine to see the future of DSiPaint and it ends up having a horrible future. Users getting banned 24/7, admins on strike, and Hull has suddenly left, leaving no source of leadership.