How do I transfer iTunes credit to Apple store credit ?

You cannot. Normally once funds are in Apple Account balance the only way to get them out again is to spend them in the media and app stores. See What you can buy with your Apple Gift Card or Apple Account balance - Apple Support but note the first column applies only to gift cards of a specific kind. You are asking about a=Apple Account balance.

From: Check your Apple ID balance - Check your Apple Account balance - Apple Support -->

"About unused Apple ID balance

You can't redeem or return Apple Gift Cards, App Store & iTunes Gift Cards, or unused Apple ID balance for cash, except as required by law. If your jurisdiction allows for refund of redeemed gift balances, you can contact Apple Support." Read the support article for more details.

The above excerpt only mentions balances from gift cards as being eligible for refunding, and only in limited places where laws may require that. Read the support article for more details.

"Redeem the gift card to your Apple ID. You can then add your Apple Account to your wallet to use the balance at the Apple Store" How can I transfer my apple gift card bal… - Apple Community

For a case where money was deposited into an Apple Cash account by mistake, a Community Specialist in this topic --> I made a deposit to my Apple account by m… - Apple Community recommends contacting Apple about your options:

Click here --> Choose your country or region - Official Apple Support 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 --> Contact Apple for support and service - Apple Support