How to replace Apple ID as phone number with an email ID?

When I purchased a new iPhone, I created an Apple account with my phone number. Moreover, I would like to replace my phone number as Apple ID with my email ID. I tried to change my Apple ID but the available option allow me to enter only a new phone number. My objective is to replace the used Phone number as Apple ID with email as Apple ID, because the test flight invitation goes to the email ID rather than a phone number.