How to cancel storage plan is not clear. Please help

Go to Settings > [your name] > iCloud > iCloud Storage. If you're using iOS 10.2 or earlier, go to Settings > iCloud > Storage.

Tap Change Storage Plan.

Tap Downgrade Options and enter your Apple ID password.

Choose a different plan.

Tap Done.

iCloud storage upgrades and downgrades - Apple Support

You can change the amount of storage by selecting a new plan right from your iPhone, iPad, iPod, Mac, or PC (for example, a 5 GB plan to replace a 200 GB plan). The downgrade will take effect after your current monthly or annual subscription ends.

iOS

1. Go to Settings > Apple ID > iCloud > Storage > iCloud.
2. Tap Change Storage Plan.
3. Tap Downgrade Options and enter your Apple ID password.
4. Choose a different plan to use when your current storage plan expires.
5. Tap Done.

OS X

1. Choose Apple menu > System Preferences, then click iCloud.
2. Click Manage in the lower-right corner.
3. Click Change Storage Plan in the upper-right corner.
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.

Windows

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

Storage upgrades and downgrades

Downgrade your iCloud storage as per this Apple Support Topic:

iCloud storage upgrades and downgrades - Apple Support

Storage upgrades and downgrades

Contact iCloud Support. You may have to try twice.

iCloud Support

How do II "cancel " monthly storage plan?

There is no downgrade option. On either icloud, or itunes even, from PC I mean. How can it be cancelled withhout the device?