how do I change my iCloud account if I don’t know the password

See the following Apple articles:

If you've forgotten your Apple ID email address or phone number

If you've forgotten your Apple ID email address or phone number – Apple Support (UK)

Use your mobile phone number as your Apple ID

Use your mobile phone number as your Apple ID - Apple Support

Averygrace Said:

"how do I change my iCloud account if I don’t know the password: I changed phone numbers and I can’t remember what number I used or the password"

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Recovering your Account:

Go Here:

• If you Forgot your Apple ID - Apple Support
• If you Forgot your Apple ID Password - Apple Support

For note...

Use These Forums' Password:

You are able to log in tot these forums just fine. So, the password to use would be no different, given it I the same account.