how to change apple id on device as i forget the existing

See: If you forgot your Apple ID  https://support.apple.com/en-us/HT201354

 You also can use Having trouble signing in/iForgot https://iforgot.apple.com/password/verify/appleid 

And you will need to see If you forgot the answers to your Apple ID security questions https://support.apple.com/en-us/HT201485

Before trying to answer your security questions is very important that you think about your questions, including your birth date, and their answers carefully and try to recall all details you can. If you don't answer them correctly enough, then you easily can lose control of your account.

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And, if you get this far, see Change your Apple IDhttps://support.apple.com/en-us/HT202667. 

And see What to do after you change your Apple ID or 

Password https://support.apple.com/en-us/HT204071 (supplementary to what the fist link says)

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.