How do I discard changes in my working copy that are not in the index?

For all unstaged files in current working directory use:

git restore .

For a specific file use:

git restore path/to/file/to/revert

That together with git switch replaces the overloaded git checkout (see here), and thus removes the argument disambiguation.

If a file has both staged and unstaged changes, only the unstaged changes shown in git diff are reverted. Changes shown in git diff --staged stay intact.

Before Git 2.23

For all unstaged files in current working directory:

git checkout -- .

For a specific file:

git checkout -- path/to/file/to/revert

-- here to remove ambiguity (this is known as argument disambiguation).