How can I unlock my iPhone if its locked with a different Apple ID?

If this is a used device, the previous owner needs to log out of iCloud or go to https://www.icloud.com/find/ and erase the device, then remove it from their account. If they won't do that, the device is useless. Apple won't unlock it.

Turn off Activation Lock - Apple Support.   *

If you are the original owner, contact Apple. Apple can remove the ID  if provided the required paperwork to include the original purchase receipt with the serial number on it. 

Request additional support for unlocking your device.    -

You used to be able to take the device, the original purchase receipt with the serial number, and a photo ID to the nearest Apple Store (if it has reopened) and have them remove the ID. This may have changed, so call the store before going to make sure they can help.

Genius Bar Reservation and Apple Support Options - Apple.   -

You are welcome. If she forgot the Apple ID password, have her enter the Apple (iCloud) ID, and click Forgot Password. See if she can get a Rescue or Notification email option. Link to contact Apple Support at the end of the 2nd article.

Apple ID - Manage  -     

Apple ID – Forgot Password  *

If she can't recover the password and she is the original owner, have her contact Apple as I posted above.