do you have extra yarn?
if YES, start to undo cast on and cut the undone yarn (its so much easier to keep the undone cast on yarn/tail trimmed..)
As you undo, slip onto a needle (it doesn't have to be the right size.. you can use a smaller one. (in fact, its easier to use a smaller one!)
after undoing, and picking up the stitches, you'll have 1 less stitch..
this is because when you 'change directions' you lose 1/2 of a stitch on each end..
-/-/-/-/-/-/-/-/-/-/-/ (the dash is the needle in right side up stitches..
but when you undo a cast on you have up side down stitches.
(turn the image upside down, and its hard to tell the difference.. (except you'll see more clearly how you lose the 1/2 stitch on each end.
(so 100 stitches becomes 99 stitches.)
then just add yarn and cast off loosely..
Next time, think about your cast on before you start.
some cast ons are naturally stretchier.. (long tail, Norwegian(aka german, german twist) and others are good choices.. as is tubular, or channel islands..
hats are good places to learn cast ons (i know 35+!)
Not to many stitches to cast on for a hat, and since the cast on frames the face, a pretty one is important.. (not just a stretchy one!)
most knitting books (and several magazines) have several cast ones.. you can begin to learn the ones that work best for you.