Hy updating from a so old version may be better FOR WINDOWS unistall first
and delete the "personal" gimp directory, before reinstall
If you use Windows easier way may be use REVOunistaller.
(or use after unistall SEARCH to find and delete your personal gimp folder ...i suggest Revo just because it may be handy to unistall also other SW, that if not would left some crap or even problematic dll behind)
This because is possible that you had installed extra plugin or script that now will be incompatible with 2.6,
For minor update (i.e from 2.6.x to 2.6.y) you may just overwrite