If you forgot the password, you'll have to go to http://iforgot.apple.com and reset it. After getting the new password, to change it for your iCloud account go to Settings>iCloud, scroll to the bottom and tap Delete Account, then sign back in using the new password. Note: when you delete the account it is only deleted from your phone, not from iCloud. It will also delete your synced data from your phone, but it will be synced back when you sign back in using the new password.
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