Outside of level-based increases, or Feats, I'm not aware of any game ability that permanently increases an ability. A case might be made for the spell Wish, although since this does not duplicate a lesser spell and is not one of the listed functions, it would require DM approval, and the wisher would suffer the attendant debilitating effects. Also, it would be well within the DM's province to go with the partial achievement text, and simply allow you to retroactively choose the ability increase in place of feat acquisition you chose at leveling time. This would be similar to the "hurl you forward in time to when your enemy is already dead" twisting of a death-dealing Wish.
Since your character is unable to cast Wish, you'd either need to pay a high level caster to do it for you (very unlikely), or find a ring of wishes (also unlikely).
If you really, really want this, I would suggest talking with your DM. Depending on how the campaign is run, he or she might allow you to re-do your leveling choice, or even to re-shuffle your ability scores. Some tables are very loose about this, particularly if you have invested time in the character but are fundamentally disappointed in the actual build.