With your guitar in tune, pluck the string, then gently press on the string above the nut. You should hear the note rise as you press, and then it should settle back as you release the pressure. If it stays high, the string is not moving smoothly through the slot in the nut. Do not attempt to adjust this yourself. A decent guitar tech or a luthier will do it right, the first time, for a lot less time and money than a botched home-repair can end up costing.