You can drive across the border and then come back, or if you tell the U.S. border guards that you're landing, they will give you a piece of paper in the office sending you back.
If you want to get an appointment in an office in less than two months, you need to tell them it's urgent and they will try to accommodate you, but going to the border is much simpler and much faster if you're anywhere near there.
Just don't make the mistake I made and arrive past 11:30 at night when all the immigration people had gone home. That didn't work.