Help to access files in Google Colab:
https://neptune.ai/blog/google-colab-dealing-with-files-2
To download the brain.graphml file: On https://github.com/fellnerm-elte/python_for_ds/blob/main/data/brain.graphml, select the Download button on the right, then right-click and "Save link as ...", or manually copy the content from the raw view to an empty file.