I had this question, too. I found the answer in the git documentation (https://git-scm.com/docs/git).
Run the command
git -C <git-working-directory> pull <git remote>
The specific command to answer this question is
git -C checkout/myApp/ pull
Note that it is important -C <git-working-directory> comes before the pull command and any additional pull options can be specified at the end of the command. In the example above, the git clone command would have setup the default remote repository ~/apps/myapp so it is not required to specify the remote repository.