This will perform a local git pull to obtain the code from the repo ~/code/MyPackage.jl. The code changes in the latter folder will not be automatically tracked. Any changes need to be committed first, and then in the virtual environment where you ran Pkg.add, you need to run Pkg.update to obtain the new code. If instead, you want to automatically track code changes in ~/code/MyPackage.jl, including any committed or uncommitted changes (or even without a git repo in the folder at all), you need to run
] dev ~/code/MyPackage.jl