How can I cancel my monthly purchase of extra storage?

Cancel upgraded iCloud storage by downgrading to the free 5GB plan. See instructions in the following Apple article: Downgrade or cancel your iCloud+ plan - Apple Support

As described in the above article, you can use any of the Apple devices or use a Windows PC with iCloud for Windows installed. If none of those options is possible, see the following from that support article:

 If you still need help, contact Apple Support.

EllenPearl wrote:

I’ve been trying to cancel my subscription for extra storage on the phone I no longer have.. Can’t find a way to solve this .. Anyone have an idea? I’ve tried phone calls, etc..

If you want to cancel a subscription from Apple - Apple Support