The branch name in Git is case sensitive. To see the names of your branches that Git 'sees' (including the correct casing), use:
git branch -vv
and now that you can see the correct branch and can verify the case-correctness in the branch name, do this:
git pull origin BranchName
where 'BranchName' is the name of your branch. Ensure that you match the case correctly
So in the OP's (Original Poster's) case, the command would be:
git pull origin DownloadManager
There are many more commands which you can learn on git commands.