Changing the size , o rfit of a kilt is very possible, the easiest way is to start by moving the buckles/strap arrangement, this is something that even a beginner sewer could probably manage. To remove pleats is something that can certainly be done, but it is a lot of work as it does mean a lot of unpicking and resewing and in truth it is better for an experienced kiltmaker to work on it.
The thing to bear in mind though is the time involved in the alterations which can sometimes almost approach the time to completely make a kilt, so the question might be whether or not it is worth all the extra expense, or whether to put the money towards a new one that fits.
It's not always an easy decision to take.