Mixing them together. Ascorbic acid is a reducing agent. That means it can turn a double bond into a single bond, among other things. So theoretically it could actually change one of the double bonds in kavain into a single bond, giving DHK, or Methysticin into DHM. (
And yes kavain is "double-bonded", and DHK is "single-bonded", not the other way around) It could also possibly attack the double bond on the lactone ring, and turn the molecule into something that is not a kavalactone (in other words, it could destroy it)
Now, how efficient these reactions might be, I actually have no idea: it might convert and/or destroy all, or some, or a tiny amount of the kavain, depending on various factors.