i now have apple tv thru my tmobile account. How do I change my billing for this so

See th information from T-Mobile at https://www.t-mobile.com/offers/apple-tv-plus-deal including the following:

I am an Apple TV+ customer and I pay monthly through Apple. How will Apple TV+ On Us work for me?

Once you redeem the Apple TV+ On Us promotion, your paid Apple TV+ subscription will pause and Apple TV+ On Us through T-Mobile will take over. Once the T-Mobile offer period is over (if applicable), pay $4.99 per month. Cancel anytime.

Sorry, I don't know. You may want to see if anything has changed on how your subscription looks on your phone at Settings > [username] > Subscriptions (I don't know that it should look different than usual but it may). If not, contact T-Mobile or Contact - Official Apple Support to see what they say.