How can I transfer my apple account balance to Apple Pay

If you mean your Apple ID balance to Apple Pay, you essentially cannot. The small exception is if some of this can be traced to residual gift card balance and you live in a place that lets you cash in on small residual amounts. Read this link:

Check your Apple ID balance - https://support.apple.com/HT202359 -->

"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 to request a refund. When you contact Apple, please include the following information:

Your Apple ID.

The refund amount.

Your current shipping address."