reinitialisation of iCloud without password
My device is an iPhone 5 with iOs 8.4
I have reinitialisated my apple account (ID and password)
Before I didn't used the iCloud.
Now I would like to use it but in the parameter of iCloud on my iPhone appears my previous account ID for which I haven't the password anymore.
How can I reinitialize that iCloud account with my my new apple account and without the password of the last account.
Thank you for your help
iPhone 5, iOS 8.4