Play low A, then high A at the proper pressure (the proper pressure is generally the lowest pressure that sounds the high A, or only slightly above that pressure). If the high A is flat relative to the low A, the reed needs to be pushed in further. If the high A is sharp, take it out, add more hemp and replace the reed so that it's not in the chanter quite as deep as it was before.
Once the the two A's match up, check the other notes. If any notes are sharp, you can "tape them up", meaning wrap tape around the chanter, so the upper part of the offending hole is covered with tape. This will flatten that particular note.
If any of the other notes are flat, I'd recommend a different reed for the chanter. Another (unattractive) option involves taping most of the holes (7 out of the 8 to be exact), but this is complicated to explain and I'm leaving that one alone! Still another option involves scraping the proper area of the reed to bring the offending notes into tune. To do this, you need about 20 years experience in the craft, and you also probably need special... spiritual powers. If you can pull this one off, you would have been burned at the stake in less modern times. Honestly, at this point it's best to try another reed.
Question - is this a reputable (Scottish) chanter? Just wondering, because you'll never get one of those Paki chanters to sound right...