Method using Google Colab only
- Download your .ipynb file
You can actually do it using only Google Colab. File -> Download .ipynb
- Reupload it so Colab can see it
Click on the Files icon on the far left:

Then Upload to session storage:

Select & upload your .ipynb file you just downloaded.
- Get your file’s path
then obtain its path (you might need to hit the Refresh button before your file shows up):

- Conversion using %%shell
Then, just as in Julio’s answer, execute in a Colab cell:
%%shell
jupyter nbconvert --to html /PATH/TO/YOUR/NOTEBOOKFILE.ipynb
The %%shell lets the interpreter know that the following script is interpreted as shell. Don’t write anything before %%shell, use a distinct cell for this.
The form of /PATH/TO/YOUR/NOTEBOOKFILE.ipynb will be something like /content/lightaberration3.ipynb.
- Your file is ready
Might need to click Refresh again, but your notebook.html will appear in the files, so you can download it:

The great thing about this is that nothing python-related has to be installed on your computer, not conda, not pip, only a browser.