how can i delete the existing iCloud cause i don't have the password

You will have to reset the password or recover the AppleID. The only way to remove it is to be able to sign your AppleID out of iCloud. (Turn off Activation Lock - Apple Support).

If you forgot your Apple ID password - Apple Support

Recover your Apple ID when you can’t reset your password - Apple Support

If necessary, if  you are the original owner, contact Apple. Apple can remove the Activation Lock if provided the required paperwork. Either take the device, the original purchase receipt with the serial number, and a photo ID to the nearest Apple Store (if it has reopened)

Genius Bar Reservation and Apple Support Options - Apple.    

or contact Apple ID Support. 

 Apple ID - Official Apple Support.    

Contact Apple for help with Apple ID account security - Apple Support.  (Link doesn’t always load)   

Get Support      Work through the screens to get a chat option. 

Apple Support schedule phone call

https://twitter.com/AppleSupport       People have posted it may take an hour or more to get a response.

Then you will have to use account recovery. You must regain access to the account in order to disable activation lock.