How to use T-Mobile YouTube TV promo if I already subscribe to it?

Short answer: You can’t.

Per the FAQs on the promo information page:

“I tried to redeem my code and it signed me in to my existing account. Are current YouTube TV subscribers eligible?

Existing YouTube TV customer are eligible. You will need to cancel your current subscription, in order to re-sign up and apply your coupon code for the discount”