- Download Pixelmon 3.0.2 from here: Pixelmon 3.0.2
- Go to your Minecraft directory. On Windows it's under \Appdata\Roaming\.minecraft
- Remove Pixelmon 2.5.x from the \Appdata\Roaming\.minecraft\mods directory
- Place the downloaded Pixelmon 3.0.2.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.