I gave my old phone to my daughter. HOw do I change the phone number

Remove your SIM card and put her SIM card in it→Remove or switch the SIM card in your iPhone or iPad - Apple Support

Thestus:

Just to be clear, you should also do everything here:

https://support.apple.com/en-us/HT201351

AND THEN setup your daughter w/ her own AppleID if she doesn’t already have one.

It is FAR better - and safer - for BOTH of you.

Don’t share a common AppleID.