You don't. Physics understanding is built over months, not days; new skills cannot be crammed at the last minute any more than just one night's worth of weightlifting right before the big game can benefit a football player.
Nevertheless, you might feel that your students aren't anywhere near ready for the exam coming up in a day or two. What can you do? How do you help?
Take a long-term approach. Instead of performing triage, instead of brutalizing yourself and your students with weekend-long review sessions and late-night tutoring, conduct a tactical retreat. Evaluate, why are your students feeling so unprepared? What can you as the teacher do next year to avoid this last-minute panic?
Thing is, thinking about next year is not your instinct. Your teenage students live in the moment. This moment says, help me now while my academic life seems to be crumbling. And you care about your students, so you help... then you and they move on in the catharsis after the exam, cleansing your collective memories of the despair and desperation youall felt this weekend. (Or worse, normalizing those feelings as a necessary and unavoidable part of academic life.)
But you cannot live in the moment. Your career teaching physics will span years if not decades. Chances are that if your class is panicked, this isn't the first year they've felt this way. By definition, it's insanity to do things the same way again and again expecting different results. So consider how you can change... starting with the first day of school.
Your students certainly feel an urgency now, with the exam looming, to pay attention to physics, to figure out difficult concepts even at the cost of significant brain energy. It is our job as teachers to create that urgency throughout the school year. Use every trick in your book, and learn new tricks: give frequent quizzes, don't let students get away with half-arsed work, do test corrections, don't answer questions during tests and quizzes, enforce the five-foot rule so each student is always writing her or his own understanding, don't slow the pace of the course for anything less than the apocalypse, give less-frequent cumulative tests rather than unit tests...
Spend the next few weeks asking your students what they might suggest youall could have done to reduce the pressure at year's end. Spend the summer building your toolbox, by reading this blog, by reading everything available on the College Board's AP Central website, by finding other physics teachers with whom to talk shop, by attending a summer institute. Come to school in August with a well-formed plan so your students spread their panic in itty bitty, barely noticeable doses throughout the year.
Be prepared, though. Teenagers live in the moment. You will get pushback as you demand more effort and engagement in the fall. It is, in fact, our job to weather that storm, to keep our students focused on long-term goals. That means some difficult conversations early on, that means some political damage control with parents, colleagues, and administrators. It will all pay off in May, when your students approach the impending AP exam with calm confidence. Then the NEXT year, the positive feedback from your well-prepared students will mean less pushback in the fall. And so on, until you have a well-understood culture of learning physics intensely but calmly, without drama or last-minute fear.