I have about a dozen of them in a variety of sizes, and if used in hardwood they
do get a little dull.  As Ed says, sharpen on the inside.  I have done it with a
file — they are soft steel or iron, and files work just fine for me.  Some of
them were ‘sprung’ when I found them, i.e., spread apart at the tips, and gently
squeezing them back towards having the outside of the legs parallel was easily
done with a vise.  These are no relation to precision tools, so precise methods
aren’t needed.

Charlie