Help me set update my Github

I have a github repository, but I don't know how to use it. Because I often hire different people to work on my project, I need to be able to share my latest files easily, using Github.

I have files on Github, but they are not on my computer, because someone else was working on them. So the current files are the ones on the server.

I need to update my Github with the latest files, taking care to do it 1) designed correctly, 2) securely, not sharing passwords.

I can download all of the files from my server to a .zip file, if that is what is needed.