It is not possible to extend the internal storage of your MacBook simply, except for maybe replacing it with a higher end SSD, but that is really difficult to do, and only possible if you don't have the maximum amount of storage for your specific MacBook. However to download a new version of macOS, I would recommend freeing up some storage on your device.
You could do this by upgrading your iCloud storage, which means that you can store photos and files on a cloud. You could also do this by deleting unused apps, photos or files. To do this simply and for free, just click the apple logo in the top left, select "about this Mac", and select Storage, and then Mangage. You'll get some quick and easy ways to delete unused files.
I hope this helps and let me know if you have any questions!