If you have bought extra storage in the last year then it cancelled your free storage. However the automated system sending the messages doesn't know you've done this. Just ignore the emails - they should stop after tomorrow.
If you need to be absolutely sure that you have indeed got purchased and not free storage:
- Choose Apple () menu > System Preferences, then click the iCloud icon.
- Click the Manage button.
- Click Change Storage Plan.
- Click Downgrade Options.
- Enter your Apple ID password, then click Manage.
What date is given here? -
If it's the same as in the picture you are on the free storage and need to upgrade it immediately. If it isn't you have purchased storage and need take no action.
If you need to purchase to continue your current storage level, the instructions are here:
http://support.apple.com/kb/HT5527
Scroll down to 'How do I purchase my current plan?' and expand the section applicable to your computer or device.