After your next brew, taste it. The sourness should be gone, replaced by a balanced cup with some sweetness and body. If it's still sour, check your water temperature—are you actually using boiling water? And are you preheating the press?

Another check: look at the grounds after pressing. If they look muddy and dark, you're over-extracting. If they look like coarse sand and the brew is weak, you're still under-extracting. Adjust from there.