If it is only a little too big, and has not previously been altered, you may be able to pull it in a couple inches by simply moving the left strap on the under apron and the right buckles on the side of the fell. The strap would go further from the edge of the underapron and the two buckles would go further toward the back of the kilt, each roughly the same distance. I have had this done by a well known kiltmaker on two kilts and its wokrs and looks fine. You may find a local tailor willing to do it or there are lots of folks here on the forum who could do it for you.

If it is more than a couple inches you are probably talking about a partial rebuild, which will require someone with kiltmaking experience to do the work properly. Again, seek help through these pages unless you have a kiltmaker close by.

I am sure you will get many other opinions and offers to help as your post ages. Good luck.

Edit---I see Robert chimed in ahead of me with the same ideas. As one of the well known resident kiltmakers I would trust his advice.