Go to <your-username>.github.io/<your-system-name>/edit.html
Edit the description.txt file (What it should look like), once you're done click save, this will copy current system into your clipboard and take you to github site,
select all, paste new version instead and hit commit changes. It's recommended to put some description of what you've changed into "Commit Message" box to make your life
easier when searching for some older version, but it's not required, select "Commit directly to the main branch" and hit "Commit changes" to make your changes appear on the site (github may take
up to a few minutes to process this change and make it appear on your site, to view it immediately go to <your-username>.github.io/<your-system-name>?v=<commit-id> where the can be found here, in this case e1bf13c), or "Create a new branch for this commit and start a pull request" and hit "Propose Changes" and then "Create pull request"
to mark them as in progress/review you can get a link <your-username>.github.io/<your-system-name>?OLD=<base-branch-name>&NEW=<compare-branch-name> where you can find base-branch-name and compare-branch-name here to highlight what's been changed and send it to your partner for review.
After review go to pull requests and the change you've made and hit "Merge pull request" to put the version directly on your site (again it make take a few minutes for them to actually appear on the main site), or go to "Files Changed" and edit file to put more changes and hit commit. Once you're done hit either "Merge pull request" if you decided to include those changes in your system or "Close Pull Request" to discard them permanently.