How do I downgrade my storage plan

Follow the steps in the support document you posted from ( Downgrade or cancel your iCloud+ plan) and select the Free 5GB plan. Make sure you backup anything you want to keep, as anything that exceeds 5GB will be deleted with no way to recover it.

By using these instructions (if you're sure you don't need that extra storage anylonger):--> Downgrade or cancel your iCloud+ plan - Apple Support

Regards

Giulio

If you go into settings, your name at the top and then click iCloud, and manage storage then change storage plan you will have the option to downgrade your storage plan.

hope that helps!