A kilt that is too big can usually be taken in at the waist about 2"-3" by moving the buckles. After that it should be taken to a kiltmaker for a re-build.
If it is too big in the hip It will need a re-build and pleats removed.
If it is too long it can be hemmed but is best done by someone who can do a blind hem, and who knows how to press pleats well.