Q: How do I access my account to downgrade?  I no longer have Apple products

Log into iCloud.com from any computer and change your settings.

While you may no longer use Apple hardware, iCloud is perfectly accessible from any windows computer and many non apple devices.

I do not believe you can downgrade your Storage plan from the iCloud.com website.

However, you should be able to downgrade from iTunes on a computer:

iCloud storage upgrades and downgrades - Apple Support