Mikolaj Barwicki upgraded the Cairo compiler to version 2.8.4 for the NethermindEth/starknet-remix-plugin repository, focusing on improving tooling compatibility and build stability. He integrated the new compiler as a submodule, updated the upgrade script, and modified the version atom to ensure seamless adoption of the latest Cairo toolchain. His work involved careful version control, modular repository management, and automation scripting using Shell and TypeScript. By addressing the upgrade in a structured manner, Mikolaj enabled a smoother upgrade path and reduced future maintenance overhead, demonstrating a solid understanding of build systems and compiler management within a collaborative development environment.