How do I downgrade my icloud storage plan please? As I upgraded a while ago but no longer need it

Read this: iCloud storage upgrades and downgrades - Apple Support

Settings > iCloud > Storage > Change Storage Plan.

Here's an official Apple support article for it:

Downgrade your iCloud storage plan

You can decrease the amount of storage by selecting a new plan right from your iPhone, iPad, iPod, Mac, or PC (for example, a 20 GB plan to replace a 200 GB plan). The downgrade will take effect after your current monthly or annual subscription ends.

iOS:

1. If you're using iOS 8, go to Settings > iCloud > Storage. If you're using an earlier version of iOS, go to Settings > iCloud > Storage & Backup.
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.

Quote from: iCloud storage upgrades and downgrades - Apple Support