First thing I would ask is whether the gun is new or used. If used, you might have other issues. If new, it's probably not a damaged gun.
Second thing I would ask is whether you've tried several different types of ammo. In my experience with the 686, accuracy can vary greatly with different ammo. Also, the gun will be most accurate with .357s over .38s.
Third question I would ask is whether you are causing the problem. Are you breathing properly? Are you flinching? Are you using proper trigger technique? etc. I don't ask you this question to insult you, but rather because you should be sure it's the gun before you start messing with the sights.
But, let's assume that there's nothing wrong with the gun or your technique. That means it's probably the sights. If it's the sights, then here's how to bring point of impact down and to the left:
Take the gun to the range and set the target out to your desired range. Use sandbags or anything else you have to stabilize the gun while you test-fire (as opposed to off-hand shooting). From the rest, fire a few shots to see where the point of impact is BEFORE YOU ADJUST THE SIGHTS.
Now, just in front of rear sight, there's an
elevation screw and, on the right-hand side just under the rear sights, there's a
windage screw. To LOWER the point of impact, you turn the
elevation screw CLOCKWISE. Turn it two clicks at a time and then fire a few shots. Repeat this process until elevation is adjusted properly.
Once the elevation is properly adjusted, it's time to work on windage. To move the point of impact to the left, turn the
windage screw COUNTERCLOCKWISE. Again, turn the screw a two clicks and then fire a few shots. Repeat the process until windage is properly adjusted.
At this point, your sights should be okay. If your sights are the only problem, then your problem should be solved. Also note that if you sight-in the gun for 25 yards, you may hit a little high at shorter ranges.
Hope this helps! BTW, this info comes from the owners manual of my own 686P.