Windows:
<workspace>\.metadata\.plugins\org.eclipse.core.resources\.projects\
Linux / osx:
<workspace>/.metadata/.plugins/org.eclipse.core.resources/.projects/
Your project can exist outside the workspace, but all Eclipse-specific metadata
are stored in that org.eclipse.core.resources\.projects
directory
As noted in the comments by tk421storm, and in Jeegar Patel’s answer:
In order for manual changes to take effect, make sure to do
File -> Refresh
afterwards.