To upgrade, first choose any method from the installation section above. But note: when you upgrade your Gnumeric version, it will look for your Python scripts in the corresponding version-named subdirectories. For example, if your scripts are in "~/.gnumeric/1.1.17/plugins", but you just upgraded to 1.1.18, you may need to rename that to "~/.gnumeric/1.1.18/plugins". If you want to keep and run several versions of Gnumeric, you'll have to copy or symlink them.
If you want the Python console, you'll also have to re-enable it, following the directions above. If you had made a local copy of the old one, make sure you don't copy or link that to the new directory. It won't work.
Find the new version with gnumeric --version, making sure to invoke the proper gnumeric.