Dumbass No More

A cousin to Took a Level in Badass, only the major change is in the character's intelligence. Tired of being thrown around, said dumbass decides to do some "brain trainin'". He (or she) will drop out of the storyline for a while. If he's the main character, this will be used to develop the other characters.

Contrast Took a Level In Dumbass.

Examples of Dumbass No More include:

Anime and Manga

Naruto went through this with training set up by Kakashi.
At the end of School Rumble Tenma went through it.
Hachi in One Piece. He's still not terribly bright but he's quite a bit smarter than the Dumb Muscle he used to be.
Dragon Ball example: Super Buu (and Majin Buu in general) is a short-witted villain, prone to impulsive acts and lacking in patience. However, after getting thrashed by Gohan, he manages to come up with a plan and absorb Piccolo (one of the most intelligent characters in the series) and Gotenks (a character at least his equal in power). After that, he stats playing mind games on his opponents, having backup plans, and overall being a ruder version of Manipulative Bastard Cell. And then he turns into Kid Buu...
Gourry Gabriev, in the Slayers anime series, is pretty much the resident Idiot Hero and seems to get stupider in each season. Evolution-R, the fifth season, starts portraying him more akin to his novel self (who is not an Idiot Hero but instead Obfuscating Stupidity for a number of reasons, including the fact he likes Lina explaining things to him / thinks she's cute when she's mad).

Comic Books

Impulse from Teen Titans got sick of being the The Fool of the team (which was mostly on account of his extreme ADHD), so he used his super speed to read the contents of an entire library. Because he also has Photographic Memory he basically became a walking encyclopedia. Unfortunately, he's more a Genius Ditz given that he's got the knowledge but not the wisdom or common sense.

Literature

This is the very end (which is also the beginning) of Invisible Man, with the title character "hibernating" and thinking over what he's done and how he could improve. Since he was the victim of a Xanatos Gambit and a Batman Gambit, he needed it.
Algorind the shiny-eyed paladin from Thornhold by Elaine Cunningham was less than bright lad raised in a monastery and then released into non-black-and-white world with a mission that wasn't what it seems. As such, he was a Butt Monkey who barely survived a completely deserved and mostly self-inflicted slapstick pinball, and ended up shrunk to the size of a mouse. In the sequel The Knights of Samular he got a clue enough to realize what's going on, spot a fallen paladin and deal with this problem, though still was easily manipulated by another party.

Video Games

A strange example of this trope exists within certain CRPG games like Fallout or Arcanum. Because it is possible to both start the game off as dumb (read: Int 1-3) and permanently increase stats without using bugs, some players may be playing a game "stupid" and then accidentally turn him back to standard dialogue choices as a "normal person."

Web Comics

The Order of the Stick After the timeskip following the battle for the Sapphire gate, Elan has gone from a total retard to... well, an idiot that still managed to make V look dumb by using illusion magic.
- He leveled his Int again in this strip, learning to use his Bard levels effectively.
  - Neutralize Poison has already come in handy, though it's still not enough.
- Elan once again demonstrates that his INT score is higher than it seems, although he's still severely lacking in WIS, having failed to notice the "Evil" part of his father's Affably Evil manner.

Web Original

Caboose from Red vs. Blue quickly became a rather extreme form of The Ditz. Come Recreation, however, he's managed to regain much of his intelligence (and sanity), to the point that he is fully capable of rational thought and even managed to repair and awaken Epsilon-Church. He's still fairly ditzy though.