how to change apple id from mobile number to email id

I don't know if that is possible. In the support documents talking about changing email addresses and mobile number Apple IDs they do not mention changing from one format to another. You'd best ask Apple.

Change your Apple ID - https://support.apple.com/HT202667 - "Follow these steps to change the email address that you use as your Apple ID."

Use your mobile phone number as your Apple ID

https://support.apple.com/HT207944

Contact Apple. Click here --> https://support.apple.com/choose-country-region Select your country, then a product. If you don't see one that handles your issue then keep experimenting with selections until you reach one that gets you a chat session or a telephone call and get the representative to redirect you.

or:

Contact Apple for support and service --> "See a list of Apple phone numbers around the world."

Click here --> https://support.apple.com/HT201232