turn off find my iphone without apple ID password
This question will likely get put on hold as a dup soon, but the best answer is really one I gave to another question. 

I had forgotten about the iPad being logged in to the same account and did not turn off findMyiPhone on it before changing the appleId.

What worked for me finally, I found here apple's KB.

Log out of iMessage on the iPad, and the log back into iMessage making sure to use the NEW appleId email address. Next go to settings and turn off FindMyiPhone, you will be prompted for a password and the old appleid will still be displayed in the request, use the password for new appleid that you just used to log in iMessage, and it will turn off find my iphone.

Once find my iPhone is off, you can log out the account completely, and update the email address.

Edit to add: re-reading the question... you might just need to know about https://iforgot.apple.com