Carefully. Very carefully. Why? Because if you just slam into a log, you're going to make like Superman and get launched. Ouch.
The trick to getting over obstacles in your path is being able to lift the wheels off the ground. This takes practice and the best place to get it is on a soft surface like grass, which won't hurt you too much if you fall. Don't do any riding though, until you've put your helmet on. (You might even consider knee and elbow pads.)
For practice, start by learning to lift the front wheel. Ride at a crawl, rise up out of the seat, keep your knees and elbows bent, move your body toward the rear of the bike and gently lift up on the handlebars. This should lift the front wheel off the ground. If not, try again. Remember to remain as relaxed as possible. You should be lifting the front wheel more with the rearward shift of body weight than by pulling on the bars. Keep working on this skill until it's easy for you to lift the front wheel and you feel in total control doing it.
Now, all you've got to master is lifting the rear wheel. This is similar to lifting the front, but you move your body weight forward, straighten your legs a little more and pull with your feet. It should feel like you're rocking up to balance on the front wheel. Obviously, you absolutely do not want to pull too hard and flip onto your face. So, remain relaxed and practice repeatedly until it feels natural to hop the rear tire off the ground.
When these two moves are perfected, put them together to get over small logs (bigger logs too), curbs and other obstacles you encounter. Rolling toward the obstacle, slow to a safe speed and lift the front wheel to clear it. The instant the front wheel is past the hazard, shift your body forward and lift the rear wheel so it, too, can clear the obstacle. Done properly, this is one smooth move.
Practice a lot and you'll be riding over downed trees in no time. Just be careful; very careful!