Q: i can't choose the downgrade option of 5GB Free on my iphone 5

Why can't you? Are you getting an error when you try?

Cheers,

GB

iCloud- Storage upgrades and downgrades

OK, exactly where and how are you attempting to downgrade it?

You should be going to: Settings>iCloud, click on your Apple ID (Account), click on your Storage Plan (the part that shows what it is and how much it is), click on Downgrade Options, type in your Apple ID password, and then click on the level you wish to go to. You will see right above the options for Downgrading when this downgrade will take effect:

Cheers,

GB