When it is a too-big as built but as designed it should fit situation, if the border is sewn together but not yet attached, consider going over some of the seams just a thread or so away from the existing.
Remember that any corrections you make are double because it is two pieces of fabric. If you just need to take in an inch or two, I prefer to restitch one set of blocks, skip the next set, and the restitch every other set. Checking often to make sure you haven't gone too far! If you need to take in more, you still have the ones you skipped before.
As Barb said, you can also use a "coping" strip to fit the top to border. Sometimes that is done on purpose, like when you have a block that is set flat, but you use a portion of that set on the diagonal for the border, you need to account for the changes in the math somehow.