It's important to always start any web page editing by Pulling changes from the existing repository. This ensures any changes made that are not currently in your local repository are pulled to your machine before you begin making edits.
Missing the step of Pulling means you may need to Rebase before completing the process of adding your changes to the repository. Rebasing a repository with conflicting changes requires a deeper understanding of git. For further information please see GitHub online documentation for assistance.
GitHub Desktop is great at picking up when new pages are available in a repository. Changes are often detected within moments of opening the program and notifications can be seen in the main display.
Any changes previously waiting to be pulled will now be in the folder on your local machine. The web pages are ready to be edited.