How to downgrade my iCloud storage plan with no Windows PC, Mac or iPhone available?

You will need a PC, Mac or iOS device : Downgrade your iCloud storage - Apple Support

If you don't have access to one you could try contacting iTunes Support and see if they can/will do so (assuming you are under the storage limit that you want to downgrade to) : http://www.apple.com/support/itunes/ww (click on your country's flag, the country where you live, and which is therefore the country on the account ; click on Contact Support towards the top right ; click on Get Help under 'Talk To Us’)