Normally, the content of files in the annex is prevented from being modified.
(Unless your repository is using [[direct_mode]].)

That's a good thing, because it might be the only copy, you wouldn't
want to lose it in a fumblefingered mistake.

	$ echo oops > my_cool_big_file
	bash: my_cool_big_file: Permission denied

In order to modify a file, it should first be unlocked.

	$ git annex unlock my_cool_big_file
	unlock my_cool_big_file (copying...) ok

That replaces the symlink that normally points at its content with a copy
of the content. You can then modify the file like any regular file. Because
it is a regular file.

(If you decide you don't need to modify the file after all, or want to discard
modifications, just use `git annex lock`.)

When you `git commit` it will notice that you are committing an unlocked
file, add its new content to the annex, and a pointer to that content is
what gets committed to git.

	$ echo "now smaller, but even cooler" > my_cool_big_file
	$ git commit my_cool_big_file -m "changed an annexed file"
	add my_cool_big_file ok
	[master 64cda67] changed an annexed file
	 1 files changed, 1 insertions(+), 1 deletions(-)

For more details on working with unlocked files vs the regular locked
files, see [[tips/unlocked_files]].