Do you mean the email address you use as your Apple ID and login? If so please follow these instructions carefully:
- Firstly, sign out of iCloud on all your devices: go to each of your devices and sign out in System Preferences (or Settings)>iCloud - 'Sign out' on a Mac, 'Delete this account' on an iOS device (this will not delete the account from the server). If you have 'Find My iPhone' etc enabled you will need to disable it first. Your synced data will disappear from your devices but will reappear when you sign back in.
- Create a new email address, for example at Yahoo or Gmail, or anywhere convenient (or you can use an existing address as long as it is not associated with an Apple ID). You can change it to another Apple address only if it's listed when you click the link to change.
- Go to http://appleid.apple.com and click 'Manage your Apple ID'. Sign in with the current ID.
- Where it says 'Apple ID' and gives your current ID email address, click 'Change your Apple ID'.
- Enter your new address and click 'Save changes'
- Sign back into iCloud on all your devices with the new ID and password. Your synced data will reappear (give it time).
I reiterate - before you start, sign out of iCloud on all your devices. Failure to do this will lead you into difficulties.