Upstream means the original repo, the Leo-editor repo on Github in this case. Origin means one's own clone, if any.
Upstream and
Origin are standard terms. Git Extensions is a Windows program, nothing to do with Python or anaconda, which I find easier to work with than just git itself (although it has its own learning curve).
Don't feel bad, I get confused about git/GitHub all the time. I'm not really the person to ask abut git questions, I'm afraid. I think I know this much: if you are using git from the command line, you would want to start by checking out the devel branch. You want to cd to the top directory where git stores the Leo-editor repository on your computer. Then
git checkout devel
That should get you the latest version of the devel branch. Then you have to make sure that Python uses it. I don't know how you have been doing that. How I do it, without Anaconda, is to set the PYTHONPATH environmental variable:
set PYTHONPATH=c:\tom\git\leo-editor
Then start Leo by running it from the command line:
python -m leo.core.runLeo
If you are not using git from the command line, someone else (or Prof. Google) will have to help you.