Q: need to downgrade my i cloud

Contact Apple here

(1144)

Without any apple device you may actually have to resort to calling them.

Contact Apple for support and service - Apple Support

but just in case here are instructions for Windows pc

1. Open iCloud for Windows.
2. Click Storage.
3. Click Change Storage Plan.
4. Click Downgrade Options.
5. Enter your Apple ID password, then click Manage.
6. Choose a different plan to use when your current storage plan expires.
7. Click Done.