How can I reset my icloud on my phone if I don't know the password?

You can't. She can see the following for help.

If you forgot your Apple ID password

If your Apple ID is locked or disabled

Turn off Find My iPhone Activation Lock

In case she has forgotten her security questions:

Reset Security Questions

Contact Apple for help with Apple ID account security

You may need to reach Apple’s Account Security department. If you cannot find your country or are not able to call, then complete this form and send it to them.

Try using Apple's iForgot service. If it says they don't have sufficient information, you need to contact Apple's Account Security team. To reach them, click hereand pick a method; if you can't use any of the ones listed, fill out and submit this form.

Change Apple ID password using find my iPhone .

On the iOS device , click on home . Scroll to next page > click on extras > you will find find my iPhone click on it , enter her Apple ID only , click on forgot Apple ID or password , click on next.

click on - answer security questions - click on the right arrow .

Date of birth , verify your date of birth to continue > fill up the credentials > click on next and same pop up window will appear .

New password ….

Verify password ….

Fill up the credentials and click on reset password , the password is changed .

See this article also https://support.apple.com/en-in/HT201487