In the game Portal 2, the mechanics of the portal gun (also known as the Aperture Science Handheld Portal Device) do not change or upgrade throughout the course of the game. This is by game design - the main focus is on puzzles and use of physics rather than on character or equipement progression. What changes and evolves is your, the player's, understanding and application of using the portals to manipulate the environment or overcome the obstacles presented. The better you get at figuring out the scenarios and rooms, the more effective your portal gun seems to be as you are improving in using it to its full potential. However, the portal gun being physically or functionally updated or upgraded within gameplay mechanics doesn't occur. That being said, the game does introduce new elements that interact with the portal gun in different ways, such as gels that can change the physics (like making things bouncy, or making you run really fast), or using portals in combination with light bridges or excursion funnels to achieve results. These aren't strictly portal gun upgrades, but they do expand on the Portal 2's gameplay, keeping it fresh and challenging. So, rather than looking for in-game ways to enhance or upgrade your portal gun, try focusing on understanding and mastering the game's mechanics and puzzles. Reading and watching game strategies online may be helpful as well to get the most out of your Portal 2 experience.