As @SolarMike has suggested in his answer, you may want to do a Factory Reset of the phone before you invest too much time and effort setting it up. A Factory Reset will avoid potential conflicts between apps or services installed or configured previously. But since you didn't ask, I'll assume you're happy to simply change the Apple ID. You may not have any issues at all; if you do, here's a link explaining how to do a Factory Reset. Note that you can perform the Factory Reset with or without your Dad's Apple ID & password.
And now, the answer to the question you asked: Note that this procedure will vary somewhat depending upon which iPhone you have. The following is for an iPhone 6, but other models will be similar.
You may change the Apple ID as follows:
1. Logout from the current Apple ID:
Settings app, and select
iTunes & App Store
- Tap the line near the top:
Apple ID: firstname.lastname@example.org
- In the
Apple ID pop-up window, tap the option
Once you've logged out, the Apple ID sign-in screen should be blank/empty.
2. Login with your Apple ID (or create a new Apple ID)
- Tap the
Sign In line where the old Apple ID was formerly located
- Enter your Apple ID and Password in the pop-up window, then tap
You should see the "spinning wheel" briefly, and then your Apple ID email will appear in the line near the top.