I think @baconator assumes you are moderately familiar with git, and it's not clear to me that you are.
"git clone" grabs a brand new copy of a project. So "git clone firstname.lastname@example.org:MycroftAI/mycroft-core.git" would create a new directory "mycroft-core" with the Mycroft core project source code. Then you can cd into that directory and run commands.
To update an existing copy, you can go into the directory with the code and do "git pull". That updates your local code with all changes from the original source since the last time you did a "git clone" or "git pull". If you get errors when you run "git pull", the simplest thing to do is run "git reset --hard ; git clean -fd" and then run "git pull" again. The git reset and git clean commands will destroy all changes you made, but if you're only copying the original source code and never changing anything that should be fine. Then you can run the makefile again to get the updated copy built.
(Edit: If you're not sure, just copy the complete directory you had to a second one. That way if something goes wrong with git, you can just delete the copy you're using and copy the other directory again.)
Good luck. git is amazing, but it's a complicated tool and time consuming to learn. I spent a lot of time doing web searches on "git how to ......" before I got comfortable with it.