To whom does the iPhone belong? Is it going to be yours or hers?
Here's the easy part:
How to create a new Apple ID - Apple Support
The not so easy and messy part:
Apr. 2018 post by ASC helper Winston Churchill on transferring data from one Apple ID to another. https://discussions.apple.com/message/33333380#message33333380
Feb. 2021 tips from Eric Root https://discussions.apple.com/thread/252468781?answerId=254653802022#254653802022
Look into using Family Sharing to continue to be able to use items purchased with one Apple ID with other Apple IDs.
Family Sharing - https://support.apple.com/HT201060
Set up Family Sharing - https://support.apple.com/HT201088
If the phone is yours, sign out of the old ID and sign into the new one. Do not delete individual content on the phone until you do this. If a photo of hers is on the phone and you delete it first and are using iCloud Photos, it will remove all copies everywhere. Sign out of iCloud on the device and that's not an issue.
Sign out of iCloud on your iPhone, iPad, iPod touch, Apple TV, or Mac - Apple Support
Set up your iPhone, iPad, or iPod touch - Apple Support