- Download Pixelmon 3.0.3 from here: Pixelmon 3.0.3
- Go to your Minecraft directory. On Windows it's under \Appdata\Roaming\.minecraft. On Mac it's under your ~/library/Application Support/minecraft folder.
- Remove Pixelmon x.x.x from the \Appdata\Roaming\.minecraft\mods directory
- Place the downloaded Pixelmon 3.0.3.zip file in \Appdata\Roaming\.minecraft\mods
- Remove directory: \Appdata\Roaming\.minecraft\database
- Remove file: \Appdata\Roaming\.minecraft\config\pixelmon.cfg
That should be it! Shouldn't take more than a few minutes.